The Opportunities module is all about money. How much, when will it close and who is the customer. In B2B mode, an Opportunity is linked to an Account as the customer. In B2C mode, an Opportunity is linked to a Contact as the customer.
The Opportunities module is the main input into the various Pipeline dashlets and dashboard charts.
On Professional and Enterprise editions, if Exchange Server Synchronization is enabled, you will see a Create Exchange Folder field. This field is user-specific and it instructs the CRM to create drop folders on Exchange Server.