Payroll Tax Look Up. 1) What is the difference between Gross Pay and Net Pay? 2) What is the Federal Tax that is withheld from my check used for? 3) What is social security tax used for and what percentage is taken out of my check for it? 4) What is Medicare tax and what percentage is taken out of my check for it? 5) What does EIT stand for (in regards to taxes) and what is another name for this tax? 6) What does LST stand for and does Tredyffrin have a LST?