The differences between active and passive verbs are explained in Reference Voice of Verbs.
A useful guideline to follow in writing is to use the active voice of a verb unless there’s a good reason to use the passive. Good reasons include the following:
1: The focus of the sentence needs to be on the receiver (the target, so to speak) of the action expressed by the verb rather than on the doer, or performer, of the action.
2: The performer of the action is unknown or irrelevant:
An elephant can’t be considered a real star unless she or he has been awarded the Golden Peanut. [The "considering" and “awarding” are done to the elephant.]
No votes for hissing director will be counted until everyone is drunk. [The counting is important, not which inebriated snake does it.]
Any unauthorized tickling will be severely punished. [Never mind by whom.]
We were finally permitted to snuggle.
If you’ve written a passive sentence, it’s useful to check whether you need to identify the person or thing that performs the action of the verb (in a prepositional phrase beginning with by):
The massage was recommended by Dr. Kneebone.
The mittens had been lost by the kittens.
In such cases, it’s good to make sure that the passive is necessary in the context of your paragraph. Presumably, in the two examples just quoted, the concern is properly with the massage and the mittens rather than the physician and the kittens. Otherwise, it’s probably best to switch to the active voice:
active: Dr. Kneebone recommended the massage.
active: The kittens lost their mittens.
Sometimes, writers use the passive voice in a state of first-draft unconsciousness and make additional errors while they’re at it:
ERROR: Bewitched by Mlle. Pachyderm’s beauty, her portrait was painted several times by Jumbo. [Bewitched by Mlle. Pachyderm’s beauty should modify Jumbo, but it feels as though it modifies portrait.]
Bewitched by Mlle. Pachyderm’s beauty, Jumbo painted her portrait several times.
ERROR: When Huck visits Jim, he is begged to free him so that they can continue their journey. [The writer means that Jim begs Huck, but as the sentence reads it's unclear who begs whom; it might even be some third person.]
When Huck visits Jim, Jim begs him to free him.