As you've probably discovered, the simple rule is that the definite article the refers to one particular thing while the indefinite articles a and an refer to an unidentified thing. Thus when we read
The critic panned the novel
we know the critic didn't like one particular novel named in his review. And for
I picked up a novel at the library
you know that I checked out some novel, but you don't know which one.
Unfortunately, things aren't always that simple. The definite article can mean the entire field of things under consideration, so for
The novel is the most popular form of fiction
you're talking about novels as an art form. The article doesn't have to associated with the superlative. For instance
The course will cover the novel from its beginnings in the early 18th century to the present.
The indefinite article may be used to refer to a representative singular to indicate the entire field as in your example
A novel is a long narrative.
Here you're not talking about a single book (like the one I checked out of the library) but any novel, and by implication all novels. In these uses, you may substitute the plural without changing the sense:
Novels are the most popular form of fiction.
Novels are long narratives.
With the "simple usage first mentioned, this isn't so. If I say
I picked up novels at the library
I checked out more than one book.