If you find an apple tree growing in a hedgerow or any other wild place, it's most likely grown from a discarded apple core. Although every bit as edible as cultivated apples, their taste can be sweet, bitter or anything in between, but as a general rule - the larger the fruits, the better they'll taste. Apples can be used for jams (especially if combined with blackberries), jellies, wines or eaten raw. As they're not from the supermarket, they'll probably have some bad bits (which need to be cut off) and could also be harbouring worms or wasps.
The dark green leaves are alternate on the stem, oval and slightly toothed and the tree can grow to a height of about 30 feet.