Counterfeiting is a very old crime -- people have been producing fake money for thousands of years. The United States Treasury continually tries to thwart counterfeiters by changing the appearance of American money, but there are still fake bills out there. If your money doesn't look right, check it thoroughly to make sure it isn't counterfeit.
Check the paper. The special cotton/linen paper should have tiny red and blue fibers embedded in it. Additionally, the ink should sit on top of the paper, making the lines and curves very distinct. The ink shouldn't look like it has sunk into the bill, and there should be no blurring.
Look closely at the number in the lower right-hand corner of any bill over $5. Move the bill up and down or back and forth. The ink should change from copper to green or green to black, depending on the angle of the light. If the color stays the same, you have a fake.
Look at the serial numbers on the bill. They should be evenly spaced and printed in the same color as the United States Treasury seal.
Buy a special pen that detects counterfeit bills and use this pen to write on the bill. The ink will turn black if the bill is fake. The ink will remain clear if the bill is real.