What is a spreadsheet? How can you test a workbook?

Please give an short definition of what a spreadsheet is, and a short but detailed explanation on how you can test a workbook and, thank you very much for any help!

Well, a spreadsheet is a computer program used chiefly for accounting, in which figures are arranged in the rows and columns of a grid.

I don't understand what the second question is.

Spreadsheet software arranges data cells in columns and rows and allows the user to do math and/or apply formulas to it. For example you could add columns A, B and C and place the result in Column D through the entire list.

A workbook is a number of related spreadsheets.

Article on testing:
http://www.resolverhacks.net/testing_spreadsheets.html