A lot depends on your purpose. Can you be more specific about what you want to do? In addition to the UsedRange property, you also have the CurrentRegion property, specialCells method, and the typical arsenal of range-defining and cell-finding statements, all of which can be used to define the range of cells you are interested in.
To clear the entire sheet, simply:
That would take care of cells with values
Here's some info on special cells if it helps.
Also check out the VBA help files on UsedRange and CurrentRegion.
(Also: type "Locate and select cells that meet specific conditions" in the help box in regular excel to understand better what Excel is doing with some of these selection options)