Groupe Francophone

1 - On is a pronoun which is often used to represent people in a general sense:

Quand on a tout, on est trop plein. -- Joseph Joubert
When you have everything, you are too full (satisfied).

2 - It also can be used to create a passive sentence. Technically, it's not a passive sentence, because on acts as the subject; however, because the perpetrator is not mentioned, it is passive in spirit:

On a bu tout le vin.
All the wine was drunk.

3 - It also is used to say 'we' in an indirect way:

On mange maintenant, ou on regarde le film d'abord ?
She we eat now, or watch the film first?

4 - Normally in written French, on becomes l'on when it follows et or a word ending in a vowel:

Quand on n'a pas ce que l'on aime, il faut aimer ce que l'on a. -- Roger de Bussy-Rabutin
When you don't have what you love, you must love what you have.