To add:
The dollar signs are to prevent the cell addresses from changing as the
formula is *copied* down a column, or across a row.
They *don't* prevent cell addresses from changing due to *inserting or
deleting* columns and/or rows.
As an example of when to use them:
Say you have a purchase order, with quantity in Column A, and unit price in
Column C.
In Column D you create a formula to calculate the total cost of the line
item.
In D2, you enter:
=A2*C2
Since you have say 15 lines on the order sheet, you want to copy this
formula down Column D without having to individually type the formula into
each line (row).
As you copy down the formula, XL increments the "relative" cell addresses in
the formula, so that you have:
=A3*C3
=A4*C4
=A5*C5
.... etc.
But, now you wish to add the tax to every line.
Say you sell to different localities, so the tax rate will vary between
customers.
You therefore designate a cell, say E2, to contain the variable tax rate,
since it might change with each P.O.
Since you're referencing this *single* tax rate cell, you don't want it to
change as you copy down your price formula.
So, you make your formula:
=A2*C2*E$2
Which copies down:
=A3*C3*E$2
=A4*C4*E$2
=A5*C5*E$2
.... etc.
If you were copying across columns, along a row, you could anchor the Column
cell reference by adding the dollar sign in front of the column reference:
$E2.
And of course, you could make both references absolute when copying in 2
directions,
$E$2.