Perhaps it's a gall of some sort. It couldn't possibly be an apple if the tree is a cherry tree.

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Here is a link that might be useful: Photobucket

I was thinking at first it could be a graft, especially if you just moved in and didn't know the origin of the plants in your yard. But the apple genus is Malus and the cherry genus is Prunus, and grafts of different genera are very uncommon, though they do sometimes exist. So the gall theory sounds like a better theory to me.

As Carol and simnia say, an apple can't grow on a cherry, so there must be come other explanation. A gall is one possibility. Another is that the tree is not a cherry but one of the crab apples with small cherry sized fruits and the larger apple is growing from below the graft. If you cut the 'normal' fruit across do you see one stone (cherry) or seeds in a star arrangement(apple)? A picture would be really helpful.

I've seen other(s) calling crabapple trees cherry trees on the internet recently so maybe that is the explanation.