1)Absolute Cell Reference

Situations arise in which the cell reference must remain the same when copied or when using AutoFill.  

Example:

In the example below, when calculating commissions for sales staff, you would not want cell B10 to change when copying the formula down.  You want both the column and the row to remain the same to refer to that exact cell.  By using $B$10 in the formula, neither changes when copied.