SUM is probably the most regularly used formula. It does exactly what it says on the tin. It adds things up.
It only applies to cells that are formatted as numeric values (including dates, percentages etc. as well as your bog-standard numbers). But if you sum a range that includes some cells containing non-numeric values, it will still work. It will simply ignore those cells in performing its calculation. Similarly, it will ignore logical values (i.e. those that contain a TRUE/FALSE value).
Range can be made up of a single range or a set of distinct ranges. If you're using a set of distinct ranges, they should be separated with commas.
=SUM(A1:Q20): this will SUM everything between columns A and Q, in rows 1 through 20
=SUM(A1,C5,D12,D15:D20): this will sum the ranges specified