Marvel publishes several different lines of comics simultaneously. Some of the different names you'll see are Marvel Univrse, Marvel Adventures, Marvel Knights, Ultimate and more.
Comic Book Rating
Each Marvel comic gets a rating: A is appropriate for ages nine and up, T+ is appropriate for ages 12 and up and Parental Advisory is appropriate for ages 15 and up. Other ratings you may see are self-explanatory.
Comic Book Format
Comics come in their single issue format as noted by the word "Comic", but they're also sometimes available in "Hardcovers" or "Trade Paperbacks" which generally collect several comics at once.
Issue 1 of 9 of the Kree-Skrull War