Radishes come in a variety of colors and shapes, round, egg-shaped, long and carrot-like; and in multi colors: red, purple, white, pink and even black. Generally, they can be categorized into two main types--summer and winter. Summer radishes are the small ones of bold red, pink, white or purple. Winter radishes are larger, and grow white, green or black--the black ones are quite pungent.
Technically, the radish (Raphanus sativus) is an edible root vegetable of the Brassicaceae family that was first grown in Europe in pre-Roman times. The Greek origin of the genus name Raphanus means "quickly appearing" and refers to the speed with which radishes germinate.
Growing period depends on the variety and season. Summer radishes more mature quickly, with several varieties germinating in 3-7 days, and reaching maturity in a few weeks. The crop can be extended by serial plantings a week or so apart. Winter varieties take longer to mature, but keep longer in storage.
For information on radish plant problems, see this UC Davis publication.
The most popular part for eating is the radish root, although the entire plant is edible and the tops can be used as a leaf vegetable, and treated like mustard greens or collard greens.