Voice is a grammatical term that describes the relationship between a subject and a verb

There are two main voices:

Active voice

The active voice is the most common: it indicates that the subject of the verb performs that action.

John golpeó la pelota. 

John kicked the ball.


Passive voice

In the passive voice, the subject of the verb undergoes the action being performed.

La pelota fue golpeada por John. 

The ball was kicked by John.


Other types of passive structures:

La pasiva refleja

This is a more relaxed passive in Spanish where "se" is placed in front of the conjugated main verb: 

La casa se vendió.

The house was sold.

 It is only possible to form a reflexive passive sentence with transitive verbs.


La pasiva de estado

Used with the verb estar, it reflects a result after something has happened.
La casa está vendida.  

The house is sold.


Voice exists in all tenses:

Active Passive Pasiva refleja (se)
Los niños leen el libro.
The children read the book.
El libro es leído por los niños.
The book is read by the children.
Los libros se leen.
The books are/are being read.
El alcalde inaugurará la nueva fábrica.
The mayor will open the new factory.
La nueva fábrica será inaugurada.
The new factory will be opened.
La nueva fábrica se inaugurará.
The new factory will be opened.
El fontanero arregló el grifo. 
The plumber fixed the tap.

El grifo fue arreglado.

The tap was fixed.

El grifo se arregló.

The tap was fixed.