Perennial with creeping underground stems which have small fibrous roots at every joint. It is very invasive and a small fragment of the rhizome will take root. If it is growing near to a boundary, it will most likely be on both sides so has to be dealt with as such. A barrier of thick plastic burried to about 30 cm should prevent ingress of the creeping stems.
The leaves and stems have a sweet taste. An infusion of the bruised roots increases urine production, is soothing and has antibiotic properties, so it is used for cystitis and other kidney and inflammatory urinary tract diseases.
It is a primary food source of the Speckled Wood Butterfly caterpillar.

Flowers May to October, producing green herringbone-like seed-heads
Height up to 1 metre.

Difficult to eradicate by cultivation, but repetition will eventually exhaust the perennial root system. It does not survive close mowing so is not a problem in a regularly cut lawn, however it can arise at the edges and in nooks-and-crannies, which are missed or ignored.
Dig out as much as possible, use a fork to avoid chopping up the rhizomes. If any do regrow carefully tease them up with a hand fork, or allow the shoots to grow to a reasonable size and spray with a herbicide. If other plants are near dig them up and tease the invading roots from the root-ball in the winter or early spring.
Never add the uprooted material to the compost heap, unless it has been allowed to dry out completely. Better to put in the recycling bin.
It is said that tomato plants or turnips grown in the infested area will inhibit the growth of the couchgrass, this is thought to be something given off by their roots which has an allelopathic effect, but whether it works or is just a coincidence or the cultivation, is not known. The Mexican Marigold (Tagetes minuta) is also known to secrete a herbicide from its roots so makes a good companion plant to try to deal with Couchgrass.
Weedkillers to use:-
Glyphosate, systemic action, taken down into underground parts.