Array formulas can help you in Excel to perform complex calculations, expecially if you want to do calculations with certain or multiple criteria.

Here are a few links that provice more information and examples on how to use them:
http://www.cpearson.com/excel/array.htm
http://www.mrexcel.com/tip011.shtml
http://office.microsoft.com/en-us/excel/HA010872901033.aspx
http://www.pcmag.com/article2/0,4149,1265433,00.asp
http://www.exceltip.com/exceltips.php?view=category&ID=106
http://tech.yahoo.com/gd/building-an-array-formula-in-excel/153304
http://support.microsoft.com/kb/275165
http://www.bettersolutions.com/excel/EDK153/LT423111411.htm