"L" Is for "Dyslexia" is all about the way the media portrays dyslexia. Generally, fictional dyslexia manifests as scrambled words and letters; the difficulty the character has in reading is Played for Laughs because of their amusing spoonerisms or Played for Drama because they're illiterate.
In reality, things are a bit more complicated. First off, people read English in two ways: "Sight words" are common words that people memorize, while "phonics" involves sounding out a word by putting together the sounds that the letters make. Some dyslexics have trouble with phonics, thus having to learn to read by rote memorization. These dyslexics thus have trouble reading words that they have never encountered before but have no more problem reading difficult or bizarrely spelled words than any other person. A person with this would be fine reading something as long as they had seen all the words before, but would not be able to figure out any word that they had not seen before. The other type are "surface dyslexics", people that can sound out words no problem, but are unable just memorize some words, thus when reading something they would go slower
then than others (they can't just skip any 3 letter word that begins with "TH") and would also always have problems with any word that wasn't phonetic. The first type (trouble sounding out words) is the one classically thought of as dyslexia, and more common; Wikipedia mentions a study by Castles and Coltheart that found phonic dyslexia in 55% of dyslexic boys. As real dyslexia doesn't actually involve seeing letters in the wrong order (that is just an "artistic representation" of the disease) there are no Spoonerisms associated with real dyslexia (a very severe form of Dyslexia known as "pure word blindness" does jumble letters around and cause illiteracy, but doesn't impair speech at all).
Real dyslexia is a spectrum: there are some people that are functionally illiterate due to it. There are a lot of others, however, that are simply very bad at reading. If you have a friend that when playing video games once spent 15 minutes looking for the entrance to the Church of the Deus Ex Machina, only for you to point out it's the door that has Church of the Deus Ex Machina above it, that person probably has dyslexia. He can't read fast and might not be able to figure out "Machina" with out doing a lot of work, so he just ignored the text.
Real dyslexia also doesn't necessarily impair writing. While most dyslexics are bad at writing, it's just a side effect. Reading is basically practice on how to spell; thus, if you don't read much, you're not going to learn the various spellings of words very well. There are no physical impairments; many dyslexics, despite being very very bad at reading, have absolutely no problem writing very well. This is different from internet dyslexics; on message boards, "I'm dyslexic" is often code for "I'm going to make no effort whatsoever to post coherently and people will White Knight if you call me on it" and is popular with trolls.
That is not to say there aren't dyslexics who are also dysgraphic (see below) — the trope for which would be the "backwards S" used to typify children and "dumb" people writing. This can be caused by a problem with sight, or not being able to recognize reflections, thus 'b' and 'd' look the same, and both hands make an 'L'. Learning disabilities are pack animals, like wolves: if you have one, you are much more likely to have another one.
On a side note, dyslexia is not as prevalent in some other countries. The Korean, Spanish, and Italian writing systems have a much more regular correspondence between sound and spelling than French or English. This makes phonics much easier to learn and sight words less critical, and studies have found less dyslexia among speakers of languages with such a regular spelling. In fact, most speakers of those languages don't even feel the need to talk about "spelling" or "dyslexia". Of course, the massive overdiagnosis of dyslexia in English-speaking countries doesn't help this bias (in the UK, students can get extra exam time if they're dyslexic: one in four applies, and although not all of them get it, many doctors no longer even ask for evidence).
In the real world there are a few other types of learning disabilities that often either get lumped together with dyslexia.
- Dysgraphia is a catch all term for disorders affecting spelling and/or writing. The Dyslexic variety causes difficulty with spelling without affecting handwriting or reading ability, though due to the wolflike nature of learning disabilities Dyslexic Dysgraphia and Dyslexia are often found together. Motor Dysgraphia is Exactly What It Says on the Tin, and is marked by really awful handwriting. The Spatial variety is caused by problems with spatial awareness and involves perfectly normal spelling skills and bad handwriting not caused by motor issues.
- Dyscalculia is a problem with math. This can range from some one who has a tendency to get mixed up when counting in their head to someone who can not comprehend any number past 3. There is a real case of a woman who, after a car accident, could not say the name of any number past 3. She could say "ate", but when she was shown the word "eight", she could try to sound it out, but that was it. Motor dysgraphia can be mistaken for dyscalculia in schools: someone who's very slow at answering written math questions may simply be slow because of difficulty writing down the answers.
- ADD/ADHD can cause people to mix up letters when writing and misread things due to the inattention it causes.
- Dyspraxia is caused by incorrect development of the area of the brain that controls coordination, and results in varying degrees of poor hand eye coordination, sequence memorisation, distance judgment, handwriting, basic maths skills and confusion between left and right. It means that puzzles can be a lot more difficult, sport becomes a lot harder and it also makes learning to drive a trial...
- Auditory Processing Disorder has a rare subtype sometimes called "Auditory Dyslexia" which causes tone deafness, poor spelling, worse listening comprehension, and the garbled speech and spoonerisms people associate with real dyslexia.
- Strong visual/spatial reasoning skills can be mistaken for dyslexia: 'd', 'b', 'p', and 'q' are the same shape in different orientations. The easiest way to identify this is if someone has little difficulty reading upside-down or mirror writing, but can't consistently name those four letters of the alphabet.
- Some types of dyslexia are related to Irlen Syndrome, in which the brain doesn't properly pick up signals from the eyes, tries to readjust, and things can look scrambled or "floating around" as a result.
Never assume that people (online or off) who claim to be dyslexic are just too damned lazy to read or write properly and are trying to garner sympathy. These conditions are extremely wide-ranging and a lot of people still just don't know where or how to get help, or assume it's just a matter of "trying harder". Until relatively recently many of the above conditions were not recognized or thought to be relatively uncommon. People who had them (especially in "milder" forms) were written off as "just lazy". They go through life believing this and struggling on their own to overcome it.