Annual, spreading by seed, it is not a grass as the common name might suggest, but is a member of the Polygonaceae Family. The leaves have a short stalk and are rounded at the ends. Sap can be irritant to the skin.
Flowers appear from June to October as a cluster of small pink flowers in the angle between leaf and stem.

Height - up to 60 cm, but on open ground it is usually prostrate, scrambling over the surface.
Hoe or hand pull; tough branching tap-root. Apply mulch to newly cultivated beds to prevent germination. The seed can lie dormant for some time, so allowing a fallow period before planting or sowing should allow germination and the weeds can be removed.

Weedkillers to use:-
Paraquat, Diquat kill on contact.

See also Common Knotgrass and Cornfield Knotgrass which have similar growth habits and flowers. Redshank is related but has flower spikes.

Follow these links for further details on Weeds, Weed Removal and Weed Prevention.