A sentence is written in the passive voice when it's action is done to, and not by it's subject. With active voice the subject is the do-er. The switch happens between object and subject.

Ex. Passive voice: The cake was made.

• The subject is "the cake". Note: the maker of the cake is missing here, which is something that happens a lot in the passive voice; the do-ers get left out. It's something that can be useful or damaging to one's writing depending on his purpose.

Active voice: I made the cake.

• The subject is “I”, and "the cake" becomes the object.

Not a convention in itself, passive voice has existed as long as written language. It's an approach more than a technique, and appears frequently in diplomatic and “official” writings precisely because it tends to water things down. Thus the government official admits that “mistakes were made”, and avoids directing the blame at himself ("I made mistakes"). Passive voice is also used in writing that is necessarily objective, such as scientific reports. In such a case the do-er, the scientist, is not as important as the thing which is being acted upon, and therefore it makes sense that the object of the experiment would be the subject of the sentence. Active voice, however, is generally preferred in rhetorical writing for its more direct approach.

Passive voice appears most frequently in the construction subject+state of being verb+action verb.

Ex. He was hit. (Active voice: John hit him.)

It also pops up in infinite verb phrases...

Ex. To run all the way would be too hard. (Active voice [though there isn't really a direct translation for this one]: I don't want to run all the way; it's too hard.)

and gerunds (a kind of ing-verb).

Ex. Being late to the party,... (Active voice: Since I was late to the party...)



For further Research:
http://grammar.ccc.commnet.edu/grammar/passive.htm
http://en.wikipedia.org/wiki/Passive_voice
http://www.unc.edu/depts/wcweb/handouts/passivevoice.html