Plants specially bred for bigger flowers or delicious fruits are often very poor growers. So buds or branches are grafted onto a vigorous genetic root stock which improves the growth of the whole plant.
Almost all fruit trees are grafted. Other common grafted plants are most Japanese flowering cherries, grape vines, roses, French lilacs, and trees with special flowers or colored leaves. Occasionally you can see the graft, where the two different plants were joined and grow together.
But if you can't see it, then how do you know whether there is a graft? When a tree has two different colored flowers on separate branches, or when most branches are weeping or curled while a few others are straight upright, it surely has been grafted. Problems arise when branches grow from below the graft or from the rootstock. They are always stronger, grow faster, and eventually take all the nutrients until the more desirable graft dies.
When a pink weeping Japanese cherry suddenly sports a white flowering upright branch, you know it is rootstock. When a beautiful tea rose has a shoot with lots of tiny thorns and different leaves, it is rootstock. Ditto when a curled hazel 'walking stick' shrub suddenly has a tall straight filbert tree growing from the center. Different fall colors are often another tipoff. Rootstock should always be removed to protect the plant. Mark the errant branch now, and prune it off in the early spring, before growth begins so that all the energy can go into the desirable fruit and flowers.