In German, as in English, there are both active and passive voices. In an active sentence, the person or thing doing the verb/action is emphasized.

The boy kicks the soccer ball.
Der Junge kickt den Fussball.

In the passive voice, by contrast, the action is the focus of the sentence, not the agent or performer of the verb.

The soccer ball is kicked.
Der Fussball wird gekickt.

In the above example, the fact that the ball is kicked is emphasized; however, the agent can still be added to the passive sentence.

The soccer ball is kicked by the boy.
Der Fussball wird von dem Jungen gekickt.

In English, the passive is formed with the present tense of the verb “to be” plus the past participle of the main verb (“to kick” → kicked), whereas, in German, the present tense of the verb “werden” (to become) is combined with the past participle. The agents are included by way of prepositions (“by” and “von”; German also uses “durch” (intermediaries or inanimate means) and “mit” (tools/instruments).

Der Baum wird durch den Blitzschlag verbrannt.
The tree is (being) burnt by the lightning strike. 

Das Blatt Papier wird mit der Schere geschnitten.
The paper is (being) cut with the scissors. 

Passiv Präsens (wichtig ist die Handlung, nicht das Subjekt, z.B. in Kochrezepten, Gebrauchsanweisungen): werden + Partizip II

If modal verbs are used in the present tense passive, “werden” goes to the end of the sentence, after the past participle of the main verb.

Er muss die Hausaufgaben machen. → Die Hausaufgaben müssen gemacht werden.

Passiv Imperfekt: wurden + Partizip II

The past tense (Imperfekt/Präteritum) of the passive voice in German is nearly the same as the present. The only changes are to the conjugated verb (either “werden” or modal verb), which have to be conjugated for the simple past, and the placement of “werden” at the end of sentences containing modal verbs.

Der Fussball wurde von dem Jungen gekickt.
The ball was (being) kicked by the boy. 

Die Hausaufgaben mussten gemacht werden.
The homework has to be done. 

Passiv Perfekt: sein + Partizip II + worden

In the spoken past (das Perfekt), there are two noticeable changes: 1) the past participle of the main verb is followed by a truncated form of “geworden” (past participle of “werden”) at the end of the sentence (there cannot be two past participles in a clause); and 2) the helping or auxiliary verb is “sein.”

Der Hund ist gefüttert worden.
The dog was fed. 

Die Hunde sind von den Kindern gefüttert worden.
The dog was fed by the children. 

Der Hund ist durch den Donnerschlag erschreckt worden.
The dog was startled by the thunder.