Actually that is a good question.

The bud is the part we want to partake of. Trichomes concentrate there and on the leaves, that is what the white snow/crystals are.

The cannabis plant has a main stalk that has many side branches that come off from each place a "sun" leaf comes out from the stalk. These in turn will grow stalks, with more stalks coming out, smaller sun leaves as you approach the top, and the little white pistils (hair) are intermixed with the very short leaves.

A 'bud' may be as small as a dime or it may be over a foot long and 2" in diameter or more for the very topmost buds.

Like any flowering plant, the bud is the part that has the flower(s). In cannabis, as you approach the top of the plant the bud becomes much more prolific and the sun leaves are totally independent. The Buds will still have leaves, but not the same size and will be part of the compact 'bud'

The first picture is two very top buds. They are comprised of many smaller buds that grow directly off the main. The entire mass is known as a cola.

Second picture has one main cola with a sub branch and nice bud on the right. Note the yellow sun leaves. At the base of each is a bud.

Third picture is the bud at the top of a cola. Note how the leaves get smaller and smaller.

The last is a good view of how the buds grow in clumps to become the colas.