If you search the internet for "novel length," you will mostly receive the standard advice that a novel should fall somewhere between 80,000 and 100,000 words.
Now, that's good advice. But I don't like to do things half-heartedly, so here is the bigger picture of length in fiction...
Short stories, incidentally, are more generally considered to range between 1,000 and 7,500 words.
Anything under this is known as a "short-short," or as "flash fiction."
Anything longer is sometimes called a "novelette" (though because this is a derogatory term - meaning fiction which is light on substance rather than length - a "long story" is a better term).
However you describe short stories, they are published both individually in magazines and collectively in book form. Novellas might be published individually as a thin book, or they might form part of a story collection.
But let's return to the matter in hand: the length of novels...
I said above that the ideal length of a novel is 80-100,000 words. Anything less than 70,000 or over 110,000 might make publishers think twice before accepting it.
Why? Because publishing is a business:
In short, 80 to 100,000 words seems to be the "sweet spot" of novel length, particularly for a first-time novelist (who always represents more of a gamble for publishers). If you do stray outside these limits...
It is much better to write a long novel than a short one.
If you absolutely insist on producing the next Animal Farm (30,000 words) or War and Peace (500,000 words), wait until later in your career when your name carries more commercial clout.
One last thing...
Although the length of fiction is genre-dependent to an extent (fantasy novels, for example, tend to be on the thicker side), sticking to the ideal novel length (about 80,000 words, in case you weren't paying attention) is still the best advice.
Writing a first novel that doesn't represent too much of an economic gamble for a publisher makes it so much easier for the publisher to say yes.
Later, when you will hopefully have a proven track record (in terms of book sales), you will be able to write your novels to whatever length you want.