An active sentence is one in which an agent or actor (subject) does something (verb) to a person or thing (object):
The professor [subject] wrote [verb] a textbook [object].
For a passive sentence, the reverse is true – the subject has something done to it by an agent:
The textbook [subject] was written [verb] by the professor [agent].
Passive voice is often used because it takes the agent out of the process, making the text sound more ‘objective’. Often, the agent is not even specified:
The ball was kicked. [by whom?]
Experiments were conducted [by whom?] to test the humidity of the cabinet.
An increased level of salinity was observed. [by whom?]
In the past, in an effort to appear objective or to avoid saying who was doing things, researchers and government staff institutionalised this style as the ‘correct’ way to write:
The study was conducted over 3 months.
The funding to the program was cut.
See Moving away from the impersonal for further discussion of this issue and how this approach is now changing.