This is Bermudagrass. It is a terrible weed that is hard to get rid of with a selective herbicide that will not kill your other grass. The selective herbicides that are available require multiple applications each year for years just to "suppress" (not kill) the Bermudagrass.
If you have a lot of it, and your lawn is not great, it is actually easier to simply kill the entire lawn with glyphosate. (Best time to renovate a lawn is in the fall.)
Read about Bermudagrass and your options on this page: http://extension.umd.edu/hgic/bermudagrass