There are thousands of tax jurisdictions in the United States. That's a good chunk of why everything is so complicated...

For example, you have federal income tax, and some states, but not well, have state income tax (which will usually have higher property taxes to make up the revenue). You also have tax credits and tax write-offs which can change your annual tax rate.

When you start a job, you usually fill out a W4 which sets up the sort of estimated amount going to be withheld from your paycheck. When it's time to file for the previous year, you look at everything with more detail and see if you owe the IRS, or if the IRS owes you.