From the Mediterranean area and Asia the genus is now in the Brassicaceae family.
The old family name was Cruciferae from the cross-like 4 petals and Brassica species are collectively called cruciferous plants.
Kew (Plants of the World Online) recognises 42 species.
Australia has 9 exotic species originally introduced as food sources but some have escaped and become weeds.
S. E. Queensland has 5 species.
Vegetables include cabbage, broccoli, turnip, mustard seed, Brussels spouts and kale.
Many of these are forms or varieties of Brassica oleracea the Wild Cabbage which is an ancient hybrid.

Almost all species are erect annual or biennial herbs but there are a few subshrubs.
Leaves at the base, usually on a petiole may form a rosette.
The blades can be undivided (entire) but are commonly deeply dissected.
The divisions may extend to the midrib (pinnatifid) or around half way to it (pinnatisect).
There is often a large terminal lobe and up to 10 smaller pairs along the midrib (lyrate).

Stem leaves get smaller and there may be none at the top.
The lower stem leaves may be on short petiole while the upper ones have none with the base clasping the stem.
The leaves may have a few simple hairs.

Terminal inflorescences are a series of flowers, on stalks or pedicels along a midrib with the lower flowers opening first.
When all the pedicels are of equal length it is a raceme and when the lower ones are longer it is a corymb.
Some species have branched inflorescences or panicles.

Flowers have 4 sepals that may be erect and lie close together or spread out.
Two are slightly larger with a sac-like base.
The 4 petals are 0.5 to 2.5 cm long and usually have a narrow claw base.
They are a pale or bright yellow or white.

There are 6 stamens with 2 opposing median pairs and 2 single laterals.
The nectaries are mainly at the base of the median stamen filaments.
The superior ovary, from 2 fused carpels has 1 locule with a septum dividing it into 2 chambers each with a single row of ovules.
The cylindrical ovary tapers into the style which has a roughly spherical or slightly 2-lobed stigma.

The fruit are a septifragal capsule that splits along the prominent septum allowing the 2 chamber walls (valves) to fall and
    release the numerous seeds.
The conical beak, up to 1 cm long usually has no seeds but occasionally it has 1 or 2.