active voice: "Traditionally used of the opposition between active and passive (see active adj. 3, passive adj. 3); however some languages, for example classical Greek, also have a middle voice (see middle adj. 4a). " OED

Example of Active Voice: The experiment yielded desirable results. 

Example of Passive Voice: Desirable results were yielded from the experiment. 

Generally, you should use active voice to make your sentences more forceful, direct, and clear. Active voice is the grammatical construction that puts the subject of a sentence as the enactor of action.

Learn More:

1. Active and Passive Voice