Belt sanders remove stock at a rapid rate. The surface they leave behind is rarely ever perfectly flat. A palm sander or oribtal sander is usually used to finish the job.
Belt sanders are beasts of burden. The motor puts all its power into the rotation of the belt. Good belt sanders will run all day without breaking down.
Entry level sanders have a short duty cycle. They can only be used for ~10minutes before the motor overheats and must be left to cool down. Professinal sanders can be used without interuption.