Users and Volunteers are almost the same thing. Not all Users are Volunteers, but all Volunteers are technically also Users (even if they are not permitted to log into VegeBoss). Volunteers are identified by having a Volunteer Start Date, and their names can be included on the SaleDay summary "in-the-box" slip.
The Users page shows a list of all Users and Volunteers registered in the system. From here you can Add new User, or click an existing name to Edit a User. This page is only available for admin users.