The links below show that, depending on your point of view, it can be regarded as a weed or as the ideal choice for a golf green. The Greek word 'poa' means 'fodder'.
It's usually quite small, 6 to 10 inches high (15 - 25 cm), with a triangular flowerhead. The individual flowers closely overlap each other. This one had pollen sacs hanging from one or two of the flowers. There are three of these pollens sacs, or anthers, on each flower, although they soon fall. Amongst them is the feathery stigma. I couldn't make out much of the leaf joint on this particular specimen but, hidden away where the leaf sheath clasps the stem, there should be a pointed silvery ligule (strap-like tissue).
Related LinksThe Poa annua Homepage; research into strains for use in golf greens at Pennsylvania State University.
Annual bluegrass; pest management and identification at the University of California.
Linnean herbarium, Swedish Museum of Natural History; take a look at dried specimens of this grass from the collection put together by Linneaus himself; the man who named the species Poa annua.