This appears to be cranesbill, one of the wild geraniums.  It can often become a weed in lawns and vacant fields.  These are annual weeds, and often die back on their own once the hot weather starts.

Since the plant is in bloom now, it will be difficult to control it.  If you use one of the weed-be-gone herbicide mixtures in early spring, when the weed is first noticed, you should be able to stop it from growing and producing more seed.  It will probably take a few years to completely eradicate it, because there's probably a good reservoir of seeds in the soil.

Also, take steps to thicken up the lawn, by overseeding in the fall, proper fertilizing and irrigation, and other recommended lawn care practices.  This will help make it harder for this and other weeds to get started.