Cats drink milk vs Cat drinks milk

"Cats drink milk" is a general statement about what cats do.

"Cat drinks milk" doesn't really make sense, unless your cat is named "Cat" (not unheard of :-), or if you're talking about someone or something else named "Cat".

"A cat drinks milk" could also be a general statement about what cats do -- "What does a cat do? It drinks milk."

On the other hand, "The cat drinks milk" is a specific statement about a particular cat.

Cats drink milk.

Means more than one cat is able to drink milk.

Cat drinks milk.

as written is not grammatical in normal usage, because a singular noun needs an article. So it should be:

The/A cat drinks milk.

The only difference is the number of cats involved.