Hairy bittercress (Cardamine hirsuta) is an exceedingly common weed in Britain, spreading rapidly and unfussily, shooting up from even the tiniest patches of soil and producing a fresh round of seeds in an astonishingly short time. In a single year several generations of bittercress can pass through their entire life cycle, each one pushing further into new territory. This happens even quicker in poor soil, in as little as three or four weeks, but they adopt a more leisurely pace in rich soil.
I don't much mind its ubiquity, myself - the entire plant is edible, and the peppery leaves are really very tasty, though the rest of the plant tends to be a little fibrous. I find them only slightly bitter, and not all that hairy. They are good in salad and sandwiches, like any cress, or you can just munch as you go when you're weeding. They are also relatively easy plants to remove, their shallow roots yielding to the gentlest of pulling, but you can expect their seeds to persist in a rather wide radius around the parent plants. This is because the ripe seed pods explode when provoked, flinging a shower of tiny seeds outwards - a shock for the easily startled gardener, and the reason the plant is also known as 'popping cress'.
A single plant can easily produce hundreds of seeds, twenty or so in each pod, so once they get a chance to start spreading, you can expect them to keep coming back - all things considered, it is not surprising that they are often brought into a garden hidden in soil purchased from a garden centre. I recommend scoffing the seedlings before they get the chance to reproduce. The seeds also become sticky when wet, allowing them to spread even further if they are able to hitch a ride on a passing animal. This and their mustardy spiciness mean you really don't want to let them get in your eyes when the pods pop.
The leaves grow in a sort of rosette, with a circle of stalks each having pairs of near-circular leaves running up each side. The plant is usually rather small, though it can grow up to 30cm high (about a foot) in the right conditions, and the white or purple-tinged flowers are only a couple of millimetres across, with seed pods growing to two or three centimetres long. It is highly frost-resistant, staying green throughout the cold months, making it a valuable source of vitamins C and A for the winter forager, as well as various minerals. Though it mainly flowers in the Spring and Summer, it has been known to flower at almost any time. It is, as I say, a notably unfussy plant.