That is [annual bluegrass, *Poa annua*][1]. It grows in lawns during the spring and causes some very nice lawns to thin out, as it grows very thick and chokes out the lawn grass. The problem with removing it is that this is a grass its 
elf, so you cannot kill it with selective weed killer. [This site][2] gives good information on the management of the grass. Do not overwater your lawn, especially in shady areas. When you water the lawn, water deeply and infrequently, as this grass is a shallow rooted water addict. Do not overdo the fertilizer as that will give these a head start over the turf. Keep your soil loose, as this species likes compacted soil. Maintaining a thick, vigorous lawn will discourage the grass from spreading.


 ![enter image description here][3]

![enter image description here][4]
  


  [1]: http://www.google.com/url?sa=t&rct=j&q=annual%20bluegrass%20wikipedia&source=web&cd=1&sqi=2&ved=0CCMQFjAA&url=http://en.wikipedia.org/wiki/Poa_annua&ei=q5hiT6TgEqb20gGqvqG0CA&usg=AFQjCNFeKYVZ58xPGcbLYC9SPcRfPS3g6A
  [2]: http://www.google.com/url?sa=t&rct=j&q=annual%20bluegrass%20control&source=web&cd=1&sqi=2&ved=0CDwQFjAA&url=http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7464.html&ei=qJhiT8unAuHs0gHMypijCA&usg=AFQjCNHbhP1xGu--doBCi1NW3jdow08XrA
  [3]: https://i.stack.imgur.com/Fci9t.jpg
  [4]: https://i.stack.imgur.com/Z9LrJ.jpg