So what are comic books?
Well, for one thing, they'd probably be more accurately named "comic magazines", but the word "books" stuck so there you go. Comic books are published on a regular basis - typically monthly - in issues of roughly 20-30 pages each.
A single issue of a comic book is roughly equivalent to a single episode of a TV show - depending on the series, it can either be its own mini-story or a small piece of a larger narrative.
Long-running comic book series, especially those from the Big Two (more on them in a bit) are typically handled by teams of creators. One creator will handle the writing, while another - or several others - will piece together the visuals. Sometimes art or writing duties will be handed off to a new creator.
An individual creator's term on a title is called a "run". For example, if "Pat Q" writes 12 issues of "Adventure Adventures," those 12 issues will be referred to as "the Pat Q run."
Where and how are comic books sold?
Most comic books today are sold in specialized comics shops, referred to collectively as the "direct market." (If you want to find a comic shop, Comic Shop Locator keeps track of most in the USA and Canada.)
Shops put their new releases on the shelves on Wednesdays, so if you're going to visit a comics shop in the middle of the week, expect to see a bit of a crowd!
Who publishes comic books?
DC and Marvel
DC and Marvel are the two biggest comic books publishers - in fact, they're sometimes referred to as the Big Two. In any given month they'll share 2/3rds to 3/4ths of the market, depending on whether you measure by how much money they make or how many issues they sell. Marvel currently has a slight lead over DC in both areas.
DC superheroes - now in stamp form!
The Big Two are best known for - and these days almost exclusively publish - superhero stories. If you can name a superhero, odds are they belong to either company: Marvel has Spider-Man, Iron Man, the Fantastic Four, and the X-Men, (among many others) while DC has Superman, Batman, Wonder Woman, and Green Lantern (among many others.) Comics starring these characters all occur within their respective company's "universe" - Superman and Batman's adventures both take place in the DC Universe, Spider-Man and Iron Man's adventures both take place in the Marvel Universe.
(However, sometimes a DC or Marvel series will take place outside the main universe. Marvel is especially well-known for this: They have the Ultimate universe, which refreshes some characters for the 21st century, as well as the kid-friendly Marvel Adventures titles.)
Marvel superheroes - now also in stamp form!
How the heck do I read these things?
Diving into Big Two comics can be daunting, partly due to one little word: continuity. Continuity basically means "the series of events that contributes to the stories we have today." All the births, deaths, relationships, political struggles, and - of-course - major battles within the DC or Marvel universe provides the backdrop for each new issue, to varying degrees.
Continuity-dense comics have a tendency to make new readers nervous, and justifiably so. It can feel a bit like trying to watch a decades-old soap opera, or starting a college-level history class halfway through the final semester.
But have no fear! Despite their reputation for confusing newbies, superhero comics can be safely eased into with the right strategy.
Many storylines, for example, can be enjoyed with only a rudimentary knowledge of the characters starring in them (much in the same way an individual James Bond movie is fun even if you haven't seen all the others.) For added convenience, these storylines will often be repackaged as graphic novels.
If you want to skip the collections and go right for the comic books, it helps to have a local comic store with helpful employees. (If they aren't helpful, well, they don't need your money!) Pick up an issue that looks interesting and ask if it's new-reader friendly. If not, ask if there are any other issues or collections that can get you up to speed. (If the employee really knows their stuff, they might even be able to give you a recap right off the top of their heads!)
All this talk about DC and Marvel leads us to . . .
The third-ish of the market that isn't eaten up by the Big Two is divided among a dozen or so independent publishers (and countless itty-bitty self-publishers.) While some of these companies have Big Two-style universes, they're much better known for their standalone titles.
There are far too many "other" publishers to go in-depth about them all, so I'll just introduce you to a handful:
Vertigo - Technically an imprint of DC, founded in the early 90s to be a home for more adult-oriented stories that didn't fit in alongside their regular titles. Best known for fantasy and mystery (and the occasional fantasy-mystery,) but from time to time they'll publish series in other genres.
Image - Also founded in the early 90s, this time by a group of freelancers working with Marvel who decided they didn't like how that company handled creator's rights and wanted to try it their own way. While initially sticking to standard (if a little offbeat) superhero fare, Image has since attracted a slew of independent creators and now has one of the most genre-diverse lineups in periodical comic books.
Dark Horse - Considered by many to be the largest independent publisher. (Though in recent years Image has given them some competition for that title.) Mostly publishes supernatural and science fiction comics, including adaptation deals with many high-profile franchises. (They handle the Buffy "Season 8" comics, as well as most of Joss Whedon's other work, and a slew of Star Wars comics.
Oni Press - Belongs on this list less and less every year, due to shifting from a comic book publisher to a straight-to-graphic-novel publisher, but still worth noting. Publishes a very wide range of genres, and in fact avoids the kinds of stories that other publishers are known for (especially superheroes.)