The $ sign is used to tell Excel that a part of a cell reference should be treated as an Absolute ($) or Relative (no $ sign) reference
In a single cell the use of $ or not makes no difference
It only comes into play when you copy the cell
The $ sign tells excel not to change the Row or Column value as the cell is copied down/up or Left/Right. Effectively the $ sign means lock the Row or Column
A1 nothing is locked, both the rows and column will change as the cell is copied
$A1 has a Locked Column but the Row can change
A$1 has a Locked Row but the Column can change
$A$1 Both the Row and Column are locked and neither will change as the cell is copied
Have a read here: http://chandoo.org/wp/2008/11/04/relative-absolute-references-in-formulas/