Some of pop culture's best-known heroes appear in Marvel comic books, and the universe is exceedingly large—enough that Marvel publishes an "Official Handbook of the Marvel Universe" to explain how everything fits together. For those new to Marvel Comics, understanding how the continuity works can make all the difference.
Major Marvel Comics
With the rise of comic book movies, staple names of the Marvel universe, such as X-Men, the Avengers, Spider-Man, and Fantastic Four, have become well known. Others, such as Deadpool, Ms. Marvel, and Dr. Strange, remain more obscure outside of comic book lore. Even the X-Men series can be complicated, given that there are several X-series featuring assorted Mutants—such as X-Force, Uncanny X-Men, X-Factor, X-Treme X-Men, and New X-Men—as well as one-offs starring individuals, such as Rogue, Gambit, Nightcrawler, Storm, and Wolverine. The roster for the Avengers is ever changing as well, with series ranging from Uncanny Avengers and Secret Avengers to Dark Avengers and Avengers Assemble.
Marvel Continuity vs. Ultimate Marvel
Marvel's comics exist in a multiverse where many dimensions and alternate timelines exist. The bulk of Marvel's comics take place on Earth 616, though hardly ever referenced in canon. The Marvel Cinematic Universe, for example, exists within a different continuity, as does the Ultimate Marvel comics, a separate imprint of Marvel launched in 2000 as a way to attract younger readers who may not want to have to understand 40 years of comic book history to start reading. Known for a more realistic reality and grittier feel, Ultimate Marvel comics do not shy away from character deaths and heroes becoming evil. Marvel has also explored alternate possibilities with its "What If" comics, which take place in alternate universe.
Marvel has re-launched and renumbered series since its earliest days, but Marvel NOW!, a 2012 initiative, began an overhaul of the entire Marvel universe following its crossover event, "Avengers vs. X-Men". In addition to several new series, it essentially rebooted the existing canon, making changes to character costumes and introducing new storylines. However, not all of its comic book series were affected to the same extent. Some titles, such as Wolverine and the X-Men and Red She-Hulk, adopted the Marvel NOW! label but continued their existing numbering. Others are completely unaffected, such as Daredevil, Gambit, and Winter Soldier.
With most of Marvel comics taking place in the same universe, one-off crossovers are not uncommon. Larger events, however, tend to launch in mini-series with tie-in issues in connected comics, not to mention prequels and prologues. In Marvel's "Avengers vs. X-Men", "Seige", and Schism" arcs, for example, events playing out in limited edition series as well as tie-in issues from existing series. Typically, Marvel releases crossover events in trade paperbacks with all the assorted issues in one place. Lastly, while DC Marvel crossovers have happened, the events are typically not in the canon.