It depends on what you're looking to do.
To reference another cell on another sheet, the syntax is Sheetname!Cell i.e. Sheet1!A1
If you are looking to sum all the occurances of XYZ happening, then the SUMIF formula would be what you're after.
If you are looking to, say, look at a table for the cost of a part, then VLOOKUP would be a good formula.
COUNTIF is another formula to check out.
Hope this is helpful. Try the help system in Excel if you would like more immediate answers and more detailed help.