What looks like half an apple?
Q: What looks like half an apple?
A: The other half.
This is a riddle in the standard Q and A format. The joke here is that the listener starts with the false assumption that both halves of an apple must look alike, and the jokester is assuming that they might not look alike. The listener's assumption is reasonable, because everything looks like itself, so one half of an apple probably would look like another half of an apple. The trick is that the joke assumes that one half of an apple is not necessarily the same as the other half.
This is actually true, if you think about it. Some apples have blemishes, and some people don't cut apples vertically down the center (instead cutting them horizontally, or diagonally). In the case of this joke, however, the apple appears to have no blemishes, and was cut appropriately to make both halves look alike.
This is a pretty dumb joke.