Although plums can be found in markets year-round, the best season for them is autumn and early winter.
What are they? There are many varieties, all with smooth, edible skin (although it's sometimes bitter and/or tough) and a stone at the centre. When ripe, the skin colours vary according to type: some remain green, others are yellow, orange, purple, red or blue/black. Some varieties are extremely juicy while others are relatively dry. Flavours range from tart to sweet.
What are the differences? Most of the juicy varieties are best eaten raw, while the drier plums are better for cooking because they give off little moisture.
What to look for: smooth skin with no bruises. The flesh should yield to gentle pressure. With some varieties, the skin has a translucent sheen - you can see without touching that they're ready to eat.
What else? If they need to be ripened, lay the plums on a sheet of newspaper (or woven mat) at room temperature out of direct sunlight, turning them occasionally. Leave space between the plums so the air can circulate and prevent mould from forming.
Certain varieties are dried, which turns them into prunes. These are thought to aid digestion. In China, there are many types of dried plums and the flavours vary widely. They are sometimes heavily salted and dried almost completely, or flavoured with tangerine peel, liquorice, star anise and/or cassia, then semi-dried until they're still soft and succulent. In Japan, green, unripe plums are made into umeboshi (pickled plums) or they're preserved by storing in rice wine.