Attached is a photograph of what appears to be a common weed, which,...
The weed in the photo looks mature. It looks like corn speedwell and there are several types of speedwell. See our website for photos and more information. One of the best identifying features are the flowers which are light blue to white. http://extension.umd.edu/hgic/speedwell
If this is not what you are referring to you may need to wait until the weed flowers and then send photos.
See our publication on control in lawns http://www.mdturfcouncil.org/admin/uploadfiles/tt-49-broadleaf-weed-control.pdf