XY-Wing
An XY-Wing uses three bivalue cells with candidate pairs {X,Y}, {X,Z}, and {Y,Z}. The {X,Y} cell sees the other two, while those two need not see each other. Whichever value the {X,Y} cell takes, at least one of the other cells must take Z, so Z can be removed from any candidate that sees both of them.
What is an XY-Wing?
In AIC terms, each bivalue cell supplies an exactly-one relation between its candidates, and the shared houses supply weak inferences between equal digits. In ALS terms, the three cells form a size-three bent almost restricted set. The candidate relations and visibility are the definition.
How to spot it
Find a bivalue {X,Y} cell. Find a bivalue {X,Z} cell that sees it and a bivalue {Y,Z} cell that also sees it. Then look for an external Z candidate that sees both Z-bearing outer cells. The two outer cells are not required to see each other.
The logic
If {X,Y} is X, the {X,Z} cell cannot be X and is Z. If {X,Y} is Y, the {Y,Z} cell cannot be Y and is Z. Therefore at least one outer cell is Z. Any external Z candidate that sees both outer cells conflicts with whichever one supplies Z and can be removed.