The apple tree, Pyrus Malus is a tree in the Rose family and the Pear genus. The exact classification of the apple tree is up for debate, with some people suggesting that Malus is an entire subgenus os its own. However, this is probably based more on the fact that the apple is so important economically, and is so distinctive culturally so that some people feel that it should not just be lumped together with the pear; rather than on any hard scientific evidence.
The apple tree, indeed, in appearence, is quite a bit like other members of its genus- the pear and (according to some) Rowan, as well as other members of its family- the Hawthorne, the Prunus species and even the Rose. The apple is low, and naturally grows as either a bush, a shrub or a tree, depending on what situation it is growing in. However, after a few thousand years of cultivation, the apple plant has been selectivly bred to be straighter, and to grow in more of a tree like shape. Even most wild apple trees are decended from cultivated species, so what a truly "wild apple" looks like is a matter of speculation.
Like other members of its family, the apple has bisexual flowers that are fertilized from April to June (in the Northern Hemisphere), and turn into fruit and ripen sometime in the autumn. The fruit can vary from an inch across in the wild to quite large in cultivated varieties. As compared to some of its cousins, the leaf system of the apple tree can be rather chaotic, as the simple leaves grow in what seems to be a whorling pattern. Also like its cousin, the apple prefers a somewhat cold climate, as it naturally ranges from the temperate climate all the way up to cold continental climates, such as Eastern Washington and the Rocky Mountains. It is not quite as cold adapted as the Rowan or Rose, however.
The apple is a very important tree economically because of its fruit, which is used both whole, as a filling in various deserts, and as a juice. However, no other part of the tree is economically useful, as apple wood, although fairly good, is usually too small and twisty to use in constructing anything.