What is the passive voice?
In a normal (active) English sentence the subject of the sentence is at the start of the sentence and the ordering is SUBJECT-VERB-OBJECT, e.g. 'Jane hit the ball'. In a passive sentence the object is promoted to the (start) head of the sentence, and the subject is relegated to the end (tail) of the sentence, e.g. 'The ball was hit by Jane.'.
Why shouldn't I use the passive voice?
General opinion is that sentences that are written in the active voice are more engaging for the reader. You can think of it as the user become more 'actively' involved in your writing. In fact, in some European languages, such as Spanish, the passive construct is almost never used. For example a Spaniard would always say 'They robbed me.', instead of 'I was robbed.'. In English, the passive voice isn't incorrect but you can often present your thoughts in a better way.
How do I recognise the passive voice?
Indicators of the passive voice are usually variants of 'to be' such as 'was', 'were', and 'been'. But beware because not all instances of these words are passive. 'I was lazy.' for instance is not a passive sentence. To be a passive sentence they need to come before a verb in the past participle. Regular verbs in the past end in -ed, e.g. mended and rented, irregular ones can have a variety of endings, e.g. hit, and thrown. A passive sentence will look like this, 'I (OBJECT) was (TO BE) hit by the ball (SUBJECT).' Passive sentences will often or but not always have a preposition such as 'by'.
Is the passive voice always with 'to be'?
The short answer is 'No'. There are constructs in English called reduced passives, or sometimes hidden passives. In this sentence the verb 'to be' is implied but not actually present. e.g. The boy, kidnapped on Monday, had brown hair.' This could be written in longer form as 'The boy, that WAS kidnapped on Monday, had brown hair.' These sentences are generally harder to spot but your brain is busy implying the passive. They can lead to some double passive sentences, e.g. 'The boy, kidnapped on Monday, was released on Tuesday.' This sentence has a double passive, one hidden and one not.