Sequels are stories that take place after the original, often with most if not all of the same cast and frequently in the same setting if not the same location. If a work gets enough sequels, it becomes a Film Serial or Serial Novel. If they turn out to be bad, it's because of Sequelitis.
There are basically two kinds of sequels, planned and unplanned. An unplanned sequel is often the result of an author discovering, much to their delight, that the original did well and readers want more stories with their favorite dashing hero. In these cases the original is very likely a self contained stand alone story that does not require having read a Prequel to understand, often leaving no hints at what a sequel may have in store (or that there would even be one). This gives the author two choices for where to take the sequel: a simple rehash of the last adventure, with new locales and villains but with the same characters (plus a new Side Kick or Love Interest as an Audience Surrogate for those who didn't see the original), or try to go for a more epic arcing story. This is done by exploring the existing mythology and backstory that the original establishes (or weaving one in wholecloth if the original didn't have one). As you can tell, these are foolproof choices.
If it's a planned sequel then the savvy author will have at least a rough draft or idea of what happens in the sequel(s), what happened in the prequel, and leave in various Sequel Hooks and Schrodingers Guns in the original to later link it with any following stories. These plot hooks will likely be vague enough that the sequel(s) aren't constrained to go in one specific direction. If you ever run into a film that has an unusual, niggling plot detail that is never solved, it may well be because the author hoped for a sequel that never happened.
The relationship between a sequel and Prequel is a close one. Like the sequel, the prequel is by definition made after the original has been released, but takes an earlier place chronologically inside the same story while the sequel takes place after. Essentially, all prequels are sequels of a sort.
For much the same reason Hollywood likes to make adaptations of existing works from other media or Remakes of older films, big film studios would love for every film they have to become a series and earn lots of cash. There are a lot of marketing advantages to this: a pre-existing fan base, is easier to write (especially if the film series is going on a novel-per-film rate) and the actors likely had a popularity boost from the original, bringing more viewers in.
See also Series Tropes.
Tropes related to sequels:
- Actionized Sequel: The sequel is more action-packed.
- Artifact Title: When a title relates to the first story, but not to the sequels.
- B-Team Sequel: A lesser team takes over production of the sequel.
- Capcom Sequel Stagnation: When a game gets released many times with small changes between versions.
- Changing of the Guard: A sequel which features or focuses on different characters from the original work.
- Contested Sequel: When fans disagree among themselves about the quality and/or canon status of a sequel.
- Dolled-Up Installment: Practice of inserting a work into a franchise which it was not originally intended for.
- Even Better Sequel: When a sequel is widely considered superior to the original, or at least to the immediately preceding installment(s).
- Fan Sequel: When fans decide to make a sequel.
- First Installment Wins: A piece of media receives one or many sequels, but the original is the one everybody loves and remembers.
- Franchise-Driven Retitling: When an earlier installment in a series is given a new name based on what the overall franchise comes to be called through sequels.
- Franchise Zombie: Franchise lasts longer than the author intended for.
- Happy Ending Override: Sequel starts out with a previous victory negated and things much worse than when they began.
- Iconic Sequel Character: A character introduced in the sequel rises to become iconic to the franchise.
- Iconic Sequel Song: A song introduced in the sequel rises to become iconic to the franchise.
- Immediate Sequel: A sequel where the plot picks up immediately after the previous part.
- Interquel: A sequel set between two previous installments; also sometimes used for a sequel that takes place during a previous installment.
- Legendary in the Sequel: When the main character from a prior work is made into a legend.
- Legend Fades to Myth: People having a skewed memory of a previous installment in the sequel.
- Meeting-the-Parents Sequel: A sequel where one of the protagonist's mommy and daddy are introduced.
- Mission-Pack Sequel: Same game, different levels.
- Non-Linear Sequel: The sequel doesn't take place after the first story.
- Numbered Sequels: When a sequel title has a number.
- Oddly Named Sequel 2: Electric Boogaloo: Weird sequel subtitle.
- Pandering to the Base: Doing what fans want instead of what the creators and executives want.
- P.O.V. Sequel: Same story as told by a different character.
- Prequel: Sequel set before the original.
- Prequel in the Lost Age: Sequel set in the time the original says has long passed.
- Refitted for Sequel: Scene intended to be in the original is moved to the sequel.
- Refusal of the Second Call: The hero of the original is available to save the day again in the sequel but refuses to help.
- Ridiculous Future Sequelisation: In the future, fictional works have a lot of sequels.
- Running the Asylum: Hired fans turn fanon into canon.
- Same Plot Sequel: When a sequel is basically a retelling of the original
- Saved for the Sequel: Plot element left to be developed in the sequel.
- Sequel Displacement: When the original installment of a series is less known than the most popular one.
- Sequel Difficulty Drop: Sequel is easier than its predecessor.
- Sequel Difficulty Spike: Sequel is harder than its predecessor.
- Sequel Episode: The plot or resolution of a Standalone Episode in a series gets revisited in a later episode.
- Sequel Escalation: Attempts to make a sequel to have elements of the previous installment, but more of them.
- Sequel First: A sequel is released in a foreign market before the first installment.
- Sequel Gap: A fairly long time passes between when an installment comes out and when its sequel does.
- Sequel Goes Foreign: The sequel is set in a foreign country.
- Sequel Hook: A hint at a future installment. May or may not be fulfilled.
- Sequel Non-Entity: Hey, they reduced the Five-Man Band to a Power Trio! What happened to the other two characters?
- Sequel Reset: The resolution from the first story is undone for the sequel.
- Sequelitis: As the number of franchise installments goes up, the quality of them goes down.
- Sequelphobic: When an announced sequel is not welcome, despite its actual quality.
- Sequel Series: A series that takes place after the original.
- Sequel Snark: A joke mocking the idea of a sequel.
- Stealth Sequel: It seems like a stand-alone work in the same universe, but it's actually a sequel or prequel.
- Stillborn Franchise: Sequels were planned but never made thanks to a commercial failure.
- Stopped Numbering Sequels: First, sequels of the series had numbers, but then not.
- Sudden Sequel Death Syndrome: People who survived the original don't make it through the sequel.
- Sudden Sequel Heel Syndrome: Someone who was good in the original suddenly becomes evil or antagonistic in the sequel.
- Surprisingly Improved Sequel: When Even Better Sequel happens in circumstances where it's even more unexpected than usual.
- Thematic Series: Sequels that don't necessarily follow the same continuity or characters.
- Third Is 3D: The third installment in a series is filmed in 3D (films) or undergoes a Video Game 3D Leap (video games).
- Title 1: The first in the series has an indication in the title that it's the first, however, a sequel might not come.
- Tone Shift: When a sequel's tone changes from the first work.
- Un Installment: A work that doesn't actually exist nevertheless has a sequel.
- Word Sequel: The sequels follow this pattern: Name, Name II, Name III, Name: Word.
- Your Princess Is in Another Castle: It looks like the story or arc has ended, then another world/villain/problem appears.