Annual Meadow-grass

A first from this taxon - the Poaceae or grasses. This one is famously ubiquitous and flowers year round. It is Poa annua or Annual Meadow-grass and is evolved to colonise any disturbed ground rapidly. It can grow and set seed in as little as six weeks; a single plant may produce up to 20,000 seeds or as few as 10; seeds may germinate immediately or can remain dormant for a long time; it is native and is naturalized around the world, being found on mountain tops in the tropics. 
The terminology for grasses is bewildering for the beginner and I struggle to key out new species. To give you a flavour here are some excerpts from the entry in BSBI Handbook No.13 - Grasses of the British Isles for P. annua: "...procumbent culms rooting at the nodes;... sheaths compressed and keeled, smooth; ligule ...blunt; blades thin, flat or folded, glabrous, often wrinkled when young, abruptly pointed or hooded at the tip. Panicle ovate or triangular, loose and open or rarely lightly contracted, the branches solitary or paired, rarely 3 together, smooth, spreading or reflexed at maturity. Spikelets ovate or oblong, 3 - 6 flowered; glumes unequal, acute at the tip, the lower lanceolate to narrowly ovate, 1-nerved, the upper elliptic or oblong, 3-nerved; lemmas semi-elliptic to ovate in profile, ...sparsely to densely hairy on the keel and lateral nerves below, rarely glabrous, blunt at the tip; palea with curled or crispate hairs on the keels, these dense or sparese, rarely absent; anthers 0.6-1.1mm." Luckily there are excellent illustrations.
The grasses, Poaceae, are the most important plant family from an economic point of view. They are part of the order Poales, which includes the sedges and rushes.