"Per diem" is Latin for "each day", and it refers to the daily monetary allowance paid by the employer to compensate a worker for living expenses while away from home on business.
It CAN be taxable. It depends on whether the employee has to prove he has offsetting expenses or not.
It almost always becomes taxable if you exceed what the IRS considers to be a temporary working arrangement. For U.S. citizens, working arrangements cease being temporary after ONE year.
For a L-1 visa holder, the time period is 183 days.
You can find the tax table for federal income taxes on page 262 of IRS Pub 17, which can be downloaded from Internal Revenue Service (http://www.irs.gov).
For GA, income taxes are $940 for the first $20,000, and 6% of every dollar over $20,000.
Social Security tax is 6.2% of every dollar up to $97,400. After $97,400, there is NO Social Security tax.
Medicare tax is 1.45% of every dollar earned, with NO cap.