1.2.4 Imperative Sentences

The imperative mood is used to give commands, instructions or requests. Imperative sentences begin with a verb in the infinitive form, without the "to" indicator. The subject is usually implied or unspoken.

(Everybody) back to work.

(infinitive form) Get away.

(infinitve form) Show me how you feel.

Sources: http://grammar.about.com/od/il/g/impersent09.htm
http://www.vocabulix.com/grammar/imperative-sentence-english.shtml