Cabbage belongs to the brassica family which also includes cauliflower, broccoli, kohl rabi, brussels sprouts and many Oriental greens. One of the oldest known vegetables, it was being eaten by the Chinese several thousand years ago.
There are many varieties, with different growing seasons, so they are available most of the year. Look for firm fresh leaves without slime or insect damage. Winter cabbages such as the red, white and Savoy should be solid and firm in the center. Stalks of all cabbages should be clean and unbruised. Although you will probably discard the outer leaves, it is better to buy cabbage with some still attached as they protect the center of the cabbage and help keep it moist.