The difference lies on where your income comes from or how much your deductions amount to.

You can fill out a 1040A/1040EZ, whether or not you are single or married, don't have income from foreign investments or a personal business and have less than the standard deductions.

If you have any of the above criteria, you have to fill out the form 1040 and it's supporting schedules.

Basically, the lesser 1040 forms are for people with income and very little deductions.