A comprehensive and in-depth answer to the question of what a novel is and what a novel can do:

A novel has three basic characteristics:

1. It is ostensibly fictional.
2. It is written in prose.
3. It is a long-form work.

Elucidations/clarifications:

1. A novel is by definition assumed to be fictional (and generally claims to be fictional (on the publication information page if not in the text of the novel itself)), though it may be drawn entirely from life, be narrated as though recounting true events, be filled with factual information of all kinds, or contain original journalism and reporting.

2. The primary method of composition in a novel is prose (though it may contain poetry), but there are no stylistic limitations whatsoever on that prose. Most novels are narrative, easy to comprehend on the level of basic action, and minimally adventurous (if at all) in terms of the visual arrangement of text, but these qualities are not inherent to the novel. (A question for another day: what is prose?)

3. A novel must be at minimum long enough to be published as a standalone volume, but it may be long enough that it requires multiple volumes. Novels range from approximately 90-5000 pages (though I’m sure there are some novels that fall on either side of that figure (average length is approx. 200-500)).

That is as much as can be definitively said about the form of the novel (note that there are no restrictions with regard to structure, content, motive, authorship, originality, etc.). As for what a novel can do (or what novels do do):

Nothing can be said about this definitively.