There are two types of apples: eating apples and culinary (cooking) apples. Eaters are sweeter, with the most interesting flavour, as their sugars are balanced by an edge of acidity. They hold their shape well in cooking, making them the best choice for a French apple tart, a tart tatin or other continental recipes, which developed in countries without a tradition of culinary apples. Examples of popular eaters are: Granny Smith, Cox's Orange Pippin and Golden Delicious.
Culinary apples are larger and more acidic. Their sourness mellows upon cooking, so you may not need to add as much sugar as you thought to a dish. A cooking apple will become more like an eater in storage because the acids lessen over time. Some apples are termed dual-purpose, and these are best for cooking when young and for eating when older. The most popular British culinary apple is the Bramley apple.