How To Reference Worksheets Inside Workbook

I do a payroll workbook for my small company. Each sheet inside the workbook is a month. Each sheet totals the monthly labor categories. I would like to keep a "year to date" total on each employee and therefore would have to reference each sheet.

Setting up a formula (or tell me where to look) on how to total worksheets inside a workbook?

Example: the workbook contains a total of 12 sheets, representing the 12 months of the year. Add "cell reference" from sheet 1 to sheet 2 to sheet 3 to reach a total, then when sheet 4 is completed it contains the cumulative totals of all 4 sheets, etc., etc. Where I can look at this "year to date total" and compare to last years "year to date total".