Impersonal Verbs are those which are only conjugated in the 3rd pers. sing, of all the tenses. They are so called because they have no person or thing as their subject.

The principal Impersonal Verbs are:

llover - llueve - to rain, it rains
granizar  - to hail
helar - hiela - to freeze, it freezes
lloviznar  - to drizzle
nevar - nieva - to snow, it snows
relampaguear - to lighten
tronar - truena - to thunder, it thunders
alborear  - to dawn
anochecer  - to grow dark
amanecer  - to dawn

Llueve.
It rains. (It is raining.)

¿Llueve mucho en Peru?
Does it rain a lot in Peru?

¿A qué hora amanece? 
At what time does it dawn (get light)?

En invierno amanece tarde. 
In winter it dawns later.

Llegué al motel al anochecer.
I arrived at the motel at nightfall.

The verbs, amanecer and anochecer may be used as personal verbs.

¿Amanece ella contenta todos los días?
Does she wake up happy every day?

Amanecimos en Madrid y anochecimos en Guadalajara.
We were in Madrid at daybreak and at nightfall in Guadalajara.

Many ordinary verbs are used sometimes impersonally.

bastar - to suffice
convenir - to suit
hacer falta - to be wanted
parecer - to appear
faltar - to be wanting
(as a personal verb, it means also "to fail")
suceder - to happen
(as a personal verb, it means also "to succeed"

Let's look at a few examples:

Basta con que tú me lo digas.
It is enough with what you tell me.

¿Qué te hace falta?
What do you lack?.

¿Cuál parece ser el problema? 
What appears to be the problem?

No conviene explotar esta mina.
It does not pay to exploit this mine.

In the sentences below, the verb is personal, and this accounts for its being in the plural. In "me bastan diez libras," "diez libras" is the subject.

Me bastan diez libras
£10 are enough for me.

Me parecen justas sus razones
His reasons seem to me to be right.

Le sucedieron muchas desgracias
Many misfortunes happened to him.

For more examples of how ordinary verbs may be used impersonally, see the lesson Impersonal Expressions

The use of haber and hacer as impersonal verbs are discussed in several other lessons on this website.

The verb valer may be used impersonally with the meaning "to be better".

Más vale tarde que nunca.
It's better late than never.

Expressions like "¿Qué se dirá?" or "¿qué dirán?" (What will people say?) may also be called impersonal uses of the verb.