Grafting is the name given to the technique of propagating trees and shrubs by joining them.
A graft is the place on the tree or bush where the two different plants have been joined together.
One plant is chosen to be the roots (rootstock) and another plant is chosen for its fruit or flower quality (scion). Where these two meet is called the graft.
Grafting is used for most fruit trees, where the rootstock will decide the eventual height of the tree as well as its vigour. Many rose bushes also have a graft. Its quite easy to find on the trunk of the tree or shrub. Look near the soil and you should see a lumpy section; this will be the graft.
It is important for the graft to be above ground, or the rootstock will take over as being the more vigorous of the two plants.