Learn English – Article usage in “a cow” vs “the cow”

It's not an article, but it's a determiner. At least, that's how Collins lists it.

Also from Collins, a determiner is:

a word, such as a number, article, possessive adjective, etc, that determines (limits) the meaning of a noun phrase, e.g. their in "their black cat"

Informally, one could say a determiner is like a article, only more powerful.

EDIT: As for the difference in meaning, there is none. The difference is pretty much only semantic. If I saw someone prowling around my neighbor's yard, I could say:

There's a robber at the neighbor's house!

or:

There's some robber at the neighbor's house!

and the meaning is the same: an unknown robber as the house next door.

When the noun that follows is singular, some and a mean the same thing. But some is flexible enough that it can be used with the plural, too:

There are some robbers at the neighbor's house!

We couldn't use a in place of some in that sentence; however, we could omit the determiner altogether:

There are robbers at the neighbor's house!

or we could use a different determiner:

There are three robbers at the neighbor's house!

1. Use definite article, when a singular noun is meant to express a whole class:

   The cow is a useful animal for mankind The cow is an animal species useful for the homo sapiens (The) cows are useful animals for farmers

Here you are talking about classes (the cow species, the mankind, the farmers): "the cows (all of them as a group) are useful for the human beings (again all of them)".

2. Use indefinite article, in the sense of any, to single out an individual as the representative of a class:

   A cow is a useful animal for a farmer A herd of cows is useful for a farmer

In the last case you are not talking about class of the cows or the farmers, but about a single, indefinite (unknown or irrelevant the actual identity) cow (or herd of cows) and farmer.