When you think about filing income taxes you generally think about the 1040 form. Thats the 'long form'. There are also two shorter versions the 1040A and the very simplistic 1040EZ. A large percent of the population doesn't do the 1040 form. I got the numbers from the IRS 2010 tax stats.
With 142.8 million filers in 2010 the breakdown is as follows:
I was actually surprised that more people don't file 1040EZ. For a lot of people taxes amount to a W2 for income and their standard deduction and exemption.