Have Tax Questions? Ask a Tax Expert for Answers ASAP
That is very important question and very complex - there is no single answer.
We should relay on several IRS publications.
The main is publication 15 - http://www.irs.gov/pub/irs-pdf/p15.pdf - see the table on the page 32 that summarizes tax application to different types of compensation.
For various types of fringe benefits - there is a similar table on the page 6 in the IRS publication 15B - http://www.irs.gov/pub/irs-pdf/p15b.pdf
Both these publication have answers on most payroll questions and should be your desk reference books.
Let me know if you need any help.