Grammatically speaking, impersonal refers to words or structures which are invariable.
Impersonal expressions are those which do not have a specific subject, as in Il est difficile à dire.
These aren't as cold as they sound - impersonal simply means that these pronouns do not change according to grammatical person; however, some of them change to agree in gender and number with the noun that they replace.
Impersonal verbs have only one conjugation: the third person singular indefinite, or il. Some of these verbs also have personal versions with different meanings, so it's important to learn to recognize impersonal verbs.
The passive impersonal is commonly used in place of the passive voice. In this construction, a non-reflexive verb is used reflexively in order to demonstrate the passive nature of the action, as in Ça se voit - That's obvious.