How are Federal taxes witheld?

Taxes are withheld depending on what you based your income and deductions on, consult your W-4
form. It may be worth also referring to your 1040-ES form.

Whats the difference between state and federal tax withheld?

The amount of tax owed is not determined solely by the amount you earn, it is a progressive tax, so you may owe different amounts depending
on whether you are married
whether you are single
whether you have any dependents or children
There are other factors to look at as well.

for more information go to IRS.gov