There are multiple branches to the AT comic line. Firstly, you have the regular series (issues 1-60 or whatever number they're on now) in which every 3-6 issues, a different storyline occurs. This is the main line and should not be conflated with the other spinoff line of a similar name (see below)
Next, you've got the spinoff series (ex; Marceline and the Scream Queens, Candy Capers, The Flip Side, Card Wars, etc.) These are side stories that span across 6 issues and are mostly(?) separate from the main comic story line unless the writers feel like making a part canon.
After that, there are the "Adventure Time Special" books, which consist of books like the yearly Halloween Spooktaculars, Winter Specials, The Adventure Time Encyclopedia, Cover Showcase, etc.
There's also the "ADVENTURE TIME COMCS" line of books. Like I said above, THIS line and the MAIN LINE of books 1-60whatever are NOT the same. The "ADVENTURE TIME COMICS" line are a group of short stories that different artists create and send in to make one entire book. Sometimes gathered they are around a particular theme ex: Marceline, PB, Friendship and they are always numbered different than the main comic line.
They are different (example)>Adventure Time #50 (main line)>ADVENTURE TIME COMICS 001 (spinoff short story line and also multiple different art styles form different artists)
Lastly, we have the Trades and Original/Graphic Novels.
The Graphic Novels are just as the name says. A novel in graphic, comic format. These babies have a single story line that span around 120-200 pages in length depending on which one you pick up. Right now there are about 7-8 of them with another coming somewhere around October 2017. If you're confused, examples of these would be Playing With Fire (First Graphic Novel and likely the one most fans and YOU have heard of), President Bubblegum, The Ooorient Express, etc.