Relative Cell Reference: Let's say you got the number
20 in
B4. In
C4 you got the number
25. In
B3 you have the formula
B4*2. The value
20 is multiplied with 2. Giving the answer
40. Now you copy the answer to the C-column, where
C4 has the number
25. The formula is now automatically changed to
C4*2. Giving the answer
50.
When copying a cell containing a formula, the cell coördinates are automatically changed.
Absolute Cell Reference: Let's say you got the number
10 in
D4.
20 in
E4. You want to add the contents of
D4 and
E4 in the cell
A1. So you use the formula
D4+E4. This gives the answer
30 (10+20). Now when you copy the cell to B1, it will adjust the formula to
E4+F4. Or if you copy it to
A2, it will be changed to
D5+E5. In this case, it's not good. You'll need to lock the formula. Select the formula in the formula bar, and press F4. The formula should now look like this:
=$D$4+$E$4. The dollar-sign indicates a locked reference. This is absolute cell reference (the opposite of Relative Cell Reference).
There is a nice tutorial about it (with pics)
here