Crab apples (Ornamental malus) are very closely related to apples, being part of the same genus Malus. The only difference between an apple and a crabapple is the size of the fruit, and it is usually considered that any apple variety with a fruit size of less than 2" is a crabapple.
Crab apple trees are grown primarily for their ornamental value. This starts in spring with a profusion of attractive blossom, which is often scented. The brightly coloured ornamental fruits hang attractively on the tree throughout autumn, providing colour in the garden and a source of food for birds. Some varieties also have attractive bronze leaves.
Most crab apples are edible - although rather unpalatable for eating fresh. However many varieties are valuable for cooking - crab apples contain large amounts of pectin, and are useful in the kitchen for making fruit jellies. Several varieties are also useful for cider blends.
The prolific blossom also makes most crab apples excellent pollinators for all other apple and cider-apple varieties - they typically produce five to ten times more pollen than a typical apple tree. The blossom is also usually more long-lasting than that of normal apples, and spans several of the mainstream apple flowering groups. Crab apples are naturally precocious and will often start producing blossom and fruit in their 2nd or 3rd years.