Without going all technical, standard deviation is a measure of how tightly a set of values clusters around the mean (arithmetic average) of those values: the smaller the SD, the less those values spread out around the mean.

Easy to calculate in Excel: the basic formula is =STDEV(number 1;number 2;...). Generally you'll replace "number 1..." with the cell references where the numbers can be found: so =STDEV(A1;A50) will give the standard deviation of the values in the cells from A1 to A50.

Easy as pi.

It's a pity that you can't just upload the spreadsheet here and insert it into your post as an attachment -- wouldn't it just raise the tone if we could all upload spreadsheets? Talk about responsible gun owners...