Excel references each cell, co-ordinate style: With absolute references, letters denote columns and numbers are used for rows. Hence B4, is the second column along (from the left) and the fourth row down.
(There is another style of cell referencing, called relative referencing)
To reference a range of cells (for example, the column of cells starting aown to A1 down to A15 would be:
A block of cells can also be referenced. For example, C4:D5 is the block of four cells C4, C5, D4, D5. When specifying a range of cells, the first cell reference is thus the top left cell in the block and the second cell reference is that of the bottom right.