Thale-cress, Arabidopsis thaliana is one of the Brassicaceae family, some of which frequent open land like my allotment. This family are told apart by their 4 petals in the shape of a cross, hence the old family name of Cruciferae. They also usually have 6 stamens and distinctive fruiting pods. There a whole group of small members of this family that are fairly small, say less than a foot high with tiny whitish flowers, and Thale cress is one of these.
This rather beautiful illustration by Stella Ross-Craig (1949) shows the characteristic features. One of these is the basal leaf rosette, the leaves of which are actually covered in many small forked hairs. As you look higher up the plant it gets progressively less hairy to the point that the flowers are without any hairs at all. The small flowers are clustered at the top of the stem and the older seed capsules are spread spiralling down the stem below. Young flower buds are held in the axis of the stem leaves. The long slightly curved seed pods are called siliqua.