Does it get little ball shaped flower clusters:
http://tomlinsonbomberger.com/wp-conten ... clover.jpg
Then it's clover.
It is actually good for your lawn. It is a legume, related to beans and peas. As such it is a Nitrogen fixer - it can pull N out of the air and stabilize it in a plant usable form. Then when you mow your lawn (assuming of course you don't bag the clippings), all the little clover clippings break down and release the N to the soil and feed your grass. They used to put clover seed in grass seed mixes, until people got so picky about lawns should look like golf courses.
And those little flowers are very good for honeybees and other beneficial insects.