Definition: A verb that expresses the action of an unspecified, generally meaningless subject.

In its narrowest sense, an impersonal verb can have no subject. In English, only one such verb — “methinks” — remains in use, and then only in literature or for effect. Impersonal Spanish verbs in this narrow sense are the various weather verbs (such as llover, to rain), conjugated forms exist only in the third-person singular (as in llueve, it is raining).

In a broader and more usual sense, however, impersonal verbs in English are those that use a meaningless “it” as the subject. The “it,” known by many grammarians as an expletive, dummy or pleonastic pronoun, is used not to provide meaning in the sentence but to provide a grammatically necessary subject. In the sentences “It snowed” and “It is apparent he lied,” “snowed” and “is,” respectively, are impersonal verbs.

In Spanish, no equivalent of “it” is used with impersonal verbs, which stand alone using a third-personsingular conjugation. An example of an impersonal verb usage is the es in “Es verdad que estoy loco” (It is true that I am crazy.)

In Spanish, sometimes plural verbs can be considered impersonal, as in a sentence such as “Comen arroz en Guatemala” (They eat rice in Guatemala.) However, in English, such a verb usage isn’t necessarily considered impersonal.

In Spanish, the hay form of haber also is considered impersonal. In translation, “there” rather than “it” is used as a dummy pronoun.

Examples: The verbs in boldface are impersonal verbs:

• It is windy. (Hace viento.)
• It is necessary to leave. (Es necesario salir.)
• It seems messages are not arriving. (Parece que los mensajes no llegan.)
• There is happiness. (Hay felicidad.)

http://spanish.about.com/od/verbs/g/impersonal.htm