We use the passive tense when
1) the subject of the verb (doer) is the unimportant
2) when we want to hide the subject (doer)
We form the passive in German by using the auxiliary verb WERDEN as the 2nd idea in the sentence and the past participle of the main verb at the end of the sentence.
E.G. present : houses are built here - Hauser werden hier gebaut.
Future : houses will be built here - Hauser werden hier gebaut
Imperfect : houses were built here - Hauser werden hier gebaut
Perfect : houses have been built here - hauser sind hier gebaut worden