The first thing you should know is that it's pronounced KELL-tic.note The second is that it isn't synonymous with Irish. The third, that it isn't synonymous with Gaelic, either.
In fact, it may not really be understood to exist, at least in the way that it's usually assumed.
And so, the basics. The modern Celtic Kingdoms cover a large part of the area of The British Isles, referring to the whole or parts of all five modern nations, as well as a part of France. These are further divided into two distinct groups, Goidelic (or Gaelic) and Brittonic. In the Goidelic group are:
- Ireland, including Northern Ireland;
- Isle of Man,
- Brittany in France
- Yr Hen Ogledd (in English: The Old North) comprising of parts of the English counties of Cumbria and parts of Lancashire and Yorkshire as well as the south of the Scottish Lowlandsnote .
The Celtic countries or nations are the areas in which the native cultures of the British Isles persisted beyond the period of Anglo-Saxon settlement, which began about the time that the Romans left, plus Brittany (settled by Brittonic speakers from south-west Britain around the same time). All the modern Celtic nations can therefore claim continuity with pre-Roman cultures; the extent to which their cultures have survived in practice, and to which they have been politically independent from England, varies enormously both over time and between the different nations. Nevertheless, each of the Celtic nations maintained political independence from the English until at least the middle ages, and maintain at least a certain degree of cultural distinctiveness from the English and French up until today, with the exception of Yr Hen Ogledd which is more of a historical region.
The crucial thing that the Celtic cultures have in common with each other is language. Celtic languages refer to a family of languages once spoken across Europe, but with two main surviving branches today: Goidelic and Brittonic, corresponding to the two groups of nations outlined above. The Goidelic languages are Gàidhlig/Scots Gaelic, which is not Scots (a Germanic language related to English and not Celtic at all); Gaeilge, the Irish word for Irish; or Gaelg/Manx, the language of the Isle of Man. The Brittonic languages are Cymraeg, the Welsh word for Welsh; Kernowek/Cornish, which is not Anglo-Cornish (a specific variety of English); Cumbric, the historic language of Yr Hen Ogledd (which might better be understood as an extinct variety of Welsh); and Brezhoneg/Breton, which was brought over to Brittany by migrating Britons in the Early Middle Ages. However, English (French in the case of Brittany) is now the de facto language of much of these areas, even if it's spoken with a strong accent. There's also a language called Ulster Scots, which is a dialect of the Scots language actually spoken in Ireland. Although these Celtic languages share common roots, in practice they are very different and for the most part not mutually intelligible, particularly across the Gaelic/Brittonic divide (by way of analogy, a Welsh speaker can no better understand Irish than an English speaker can German, even though English and German share a common Germanic root).
Celtic, broadly speaking, can refer to the language, culture, heritage, and people pertaining to the Celtic nations. So you can be Celtic and not Gaelic, but you can't be Gaelic and not Celtic.
In practice, however, other than the common root of their languages and a few very occasional aesthetic features like Celtic crosses (such as the one depicted above)there is little in common between these groups. These are two groups of very different nations with their own histories, cultures, and literatures, and lumping them together is a modern practice - starting in the nineteenth century with the foundation of academic disciplines like linguistics and ethnography - rather than one which has a real historical basis. Welsh texts before the nineteenth century, for example, do not express any sense of common "Celticness" with the Irish, and vice-versa; and when it comes to what's frequently referred to as Celtic Mythology we're really talking about two largely unrelated traditions: that of Old Irish, and that of old Welsh, the latter including the Literature/Mabinogion, from which Arthurian Legend originated.
Even sticking with language, the picture isn't that simple: modern Scotland has always been a hybrid Germanic/Celtic culture, with Gaelic-speaking highlands and islands combined with lowlands which spoke a Germanic language, Scots, closely related to English. Whilst all the Celtic languages are now minority languages even within their own historic territories, Irish and (especially) Welsh are in relatively rude health, with many thousands of regular speakers including many children. Gaelic and Breton have fared less well, though they are clinging on in restricted ranges with some promising shoots. Cornish and Manx were effectively extinct by the mid-twentieth century, though revivals are underway in both thanks in part to earlier efforts to record the languages. Each now has maybe a few hundred speakers. Cumbric may have persisted into the 1200s, however, was not written down or directly recorded (which hasn't prevented attempts at revival) and may perhaps better be considered to be a form of Welsh (although they were not written down until centuries later in Wales, many of the oldest texts in Welsh have their origins in Yr Hen Ogledd - the name of which is Welsh and means The Old North).
Time Marches On however, and after two hundred years of being treated as if it did exist, the concept of a common Celtic-ness binding these nations together has taken on a mind of its own, with concepts like Celtic Art, Celtic Music etc.; even if most of its manifestations in practice draw only on Irish traditions. This is likely due firstly to the prominence of Irishness in the popular cosciousness of both England and the US, and secondly due to the Language Barrier meaning Welsh (which has retained its langauge better than the others) is less familiar and accessible to English-speaking culture. All this has led to the common misconception that Celtic, Gaelic and Irish all mean the same thing, as seen with e.g. American sports teams using the terms this way, as well as things like the Irish bands Celtic Thunder and Celtic Woman.
The fact that Scotland and Ireland are more closely linked with their Gaelic history is probably the reason they are often conflated, though doing this too much in Real Life might turn into a Berserk Button. A similar reaction will be received if you too often call anyone Celtic "English"note — even those living in England may prefer prefer British or Cornish/Northern. Wales and Scotland may also get mixed up (or less commonly Wales/Ireland). To make things even more complicated, increasing numbers of Scots and Welsh in particular do not identify with the term British - which the Welsh actually used to refer to themselves (i.e. the original Britons) right up until the 19th century, by which time the term had been appropriated from them (by the English) to refer to the whole island of Britain. The Irish would never want to be referred to as British - unless of course they are the unionist community in Northern Ireland. See? It's all very simple.
The Geography of the Celtic lands is famously rugged — Meaning mountainous, arboraceous, cold, and wet. However, it is very verdant: Yorkshire and County Wicklow are known as "God's Own" because of their pure green mountains and untouched natural Eden-esque appearance. Yes, everything is green. That is, when everything isn't grey: because of the mountains and the overcast nature, expect grey rain and endless fog to be the sight greeting you there. People from these areas are going to be very hardy, and proud, as well as likely poetic at how beautiful their home is.
You've probably noticed that Cornwall, Cumbria and Yorkshire are all parts of England. Yr Hen Ogledd, of which the latter two were part, was in three primary kingdoms: Ystrad Clud to the North (Scottish Lowlands), Rheged in the middle (Cumbria) and Elmet to the South (Yorkshire). In the 600s, Northumbrianote struck an invasion through Elmet and forced a political marriage with the rulers of Rhegednote . Thus, the Old North was absorbed into Northumbria, one of the Saxon kingdoms which became a part of modern England. Cornwall held out a bit longer, being invaded by Wessex and then conquered by the crown during the 800s.
- Isle of Man
- Northern Ireland
There are a lot of myths and legends, as with any ancient enough civilisation, and many are still reasonably well-believed today (even if the locals don't always want to admit they believe in them). The most famous is probably that of Arthurian Legend, set predominantly in Wales and Yr Hen Ogledd (Camelot supposedly being in present-day West Yorkshire).
The mythology of the region includes the origin of the Leprechaun, Banshee, and The Fair Folk, making any mysterious bit of nature stumbled upon (which is a lot) appear to be a portal to the Land of Faerie.
Celtic music is a subset of Folk Music, with a lot of strings and pipes used — including the local Celtic harp and bagpipes. It is often known as "trad", as in the 'trad tunes' that Ed Sheeran raps about in his Celtic fusion song 'Galway Girl'. The sounds will be designed to take you to a place on the open moors or mountains. You may well get people dancing along with their Irish dancing, Morris dancing, Highland dancing, May dancing, and whatever that thing the Scots do around an X on the ground. May be performed at a cèilidh.
Tropes relating to Celtic music and dance:<!—index—>
Celtic music artists:
- Afro Celt Sound System
- Celtic Thunder
- Celtic Woman
- The Corrs
- Nick Drake
- Gaelic Storm
- Loreena McKennitt
- Brian McNeill
- Van Morrison
- Mumford & Sons
- Sinéad O'Connor
- Ed Sheeran
A very non-comprehensive list of works of fiction which not only depict these lands but with an emphasis on their national identity.
- Tríd an Stoirm
- The Secret of Kells
- Song of the Sea
- Finnegans Wake
- Once (no relation to the film)
- Ross O'Carroll-Kelly
- The Warlord Chronicles, a "realistic" reimagining of King Arthur as a Celtic warlord during post-Roman Britain
- The Wasp Factory
- Wuthering Heights
- Cadfael (Literally borderline; the setting, an abbey in Shrewsbury, is in England, but it’s only 12 miles to the Welsh border, so many stories involve Wales and Welsh culture. Also the lead character is Welsh.)
- C'Mon Midffîld!
- Doc Martin
- Gavin & Stacey
- Moone Boy
- Mrs. Brown's Boys
- Pobol y Cwm
- Y Gwyll (Hinterland in English)