Vosotros is primarily used in Spain and is used as an informal second-person plural, i.e.: used in a group setting, where you are familiar / friendly with the people in the group [think: 'you all' / 'y'all'].

Ustedes is used more predominantly in Latin America and can be used for both informal and formal settings. 

Note: Ustedes can also be used in Spain but is reserved for formal situations, e.g.: your boss at work, or strangers.