What is an account team?

Account teams are used to grant access to accounts and their related contacts, opportunities, and cases. They are good for use in a private sharing model where typically users can't see or edit other accounts.

You can also use them to identify who works with you on an account using the roles. For example, you could have an Account Executive, Sales Engineer, etc who all work on an account.

There's a lot of documentation out there on Account Teams that a simple Google search will find: https://help.salesforce.com/apex/HTViewHelpDoc?id=accountteam_def.htm&language=th