There are two ways to refer to comics. As an art, comics are sequential images and other symbols deliberately juxtaposed to portray meaning and evoke an aesthetic response, to paraphrase Scott McCloud. Then there are the many forms comics take. These forms or formats differ from each other on levels of material, distribution methods, and content.
Comic books refer to the serialized, print periodicals, episodically portraying the narrative of a character or group. Initially their content only consisted of popular comic strip reprints, nothing original. However, following the commercial success of Superman narratives in Action Comics #1 in the late 1930s, comic books began to contain original material.
Graphic novels offer self-contained narratives, usually, released and distributed all at once in single volumes. While no one disagrees that graphic novels are lengthier in content when compared to comic books, some scholars and fans suggest graphic novels typically portray more mature themes than comic books. Thus "graphic novel " has taken on a cultural or artistic status-signifier.
In Japanese, Manga (漫画) refers to all cartoons and comics. However, in English speaking markets, Manga primarily refers to comics from and initially made for Japanese culture. They are as much a format as they are a genre of comics. They are typically lengthier than American comic books, but enjoy a serialized publication. Manga is read in East-Asian style: right to left, beginning at "the back" of the book.
Garfield, Peanuts, and Calvin & Hobbes: These are your classic single or double-strips of comic panels commonly associated with American Newspapers. They are typically very brief, and, to some, very funny!
Not to be confused with digitized comics, these are comics created, distributed, and archived on websites or social media platforms. They are typically shorter in length compared print comics. Some creators take advantage of the digital platform to make their comics more interactive.