Checked what I remembered from school on Google:

http://en.wikipedia.org/wiki/Impersonal_verb
Impersonal verb
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An impersonal verb is a verb that cannot take a true subject, because it does not represent an action, occurrence, or state-of-being of any specific person, place, or thing. Verbs indicating weather, such as to rain, are often impersonal.

In some languages, such as English, French and German, an impersonal verb always takes an impersonal "dummy pronoun" (it in English, il in French, es in German) as its syntactical subject:

It snowed yesterday.
Il a neigé hier. (French)
Es schneite gestern. (German)

In some other languages (necessarily null subject language and typically pro-drop languages), such as Portuguese, Spanish and Italian, an impersonal verb takes no subject at all, but it is conjugated in the third-person singular, which is much as though it had a third-person, singular subject:

Nevó ayer. (Spanish)
Nevou ontem. (Portuguese)