The active voice takes the form of "A does B"; the passive takes the form of "B is done [by A]."
There are two problems with the passive voice. The first is that sentences often become dense and clumsy when they're filled with passive constructions. The more serious danger of the passive voice, though, is that it lets the writer shirk the responsibility of providing a subject for the verb. Dan White gives an example:
"I'm sorry that the paper was poorly written."
If you're going to apologize, apologize: "I'm sorry I wrote a bad paper."
The active voice forces one to be specific and confident, not wimpy.
And the stakes can be higher when you're talking about atrocities worse than bad papers. This is why nefarious government and corporate spokesmen are so fond of the passive voice: think of the notorious all-purpose excuse, "Mistakes were made." Then think about how much weaseling is going on in a sentence like "It has been found regrettable that the villagers' lives were terminated" -- notice especially how the agency has disappeared altogether. It should make you shudder.
In your own writing, therefore, favor the active voice whenever you can. Instead of the passive "You will be given a guide," try the active "We will give you a guide" -- notice the agent ("We") is still there.
Don't go overboard, though. Some passives are necessary and useful. In scientific writing, for instance, sentences are routinely written in the passive voice; the authors are therefore given less importance, and the facts are made to speak for themselves. Even in non-scientific writing, not all passives can be avoided.
Don't confuse am, is, are, to be, and such with the passive voice, and don't confuse action verbs with the active voice. The real question is whether the subject of the sentence is doing anything, or having something done to it.
"I have been giving" is active, while "I have been given" is passive.