There are different 1099's, the most common is an interest statement, the other is one you get for any self-employment income that the person paying you can write off as an expense and reports what they paid you to the IRS.
For example, let's say I'm a building contractor. I hire Joe Blow's Excavating Service to dig a foundation. I pay Joe Blow $3000. When I do my taxes at the end of the year, I claim the $3000 as a business expense, send the IRS a copy of that 1099 and all the other 1099's I sent out, plus a copy to Joe Blow. Now the IRS knows that Joe Blow got $3000 and that $3000 had better show up on his tax return. Further, when I send someone a 1099 that acknowledges that I did not withhold Federal and Social Security taxes from them and they have to pay that themselves. Which is actually your Federal wage tax and double Social Security, or 15.3%, because when you work for someone else you are only paying half of your SS (7.65%) and your employer pays the other half. When you work for yourself you pay both ends of SS, or 15.3%. Plus your wage tax.