Naked Pairs
A Naked Pair is two cells in one row, column, or box whose combined candidate set contains exactly two digits. The two cells reserve those digits, so the digits can be removed from other cells in that house.
What is a Naked Pair?
Count distinct candidates in the union of the two selected cells. The union must contain exactly two digits; the two cells need not hold the same two candidates.
How to spot it
Within one house, find two unsolved cells whose combined candidates are limited to the same two-digit union. Then look outside the selected cells, but within that house, for occurrences of either digit.
The logic
Two cells need two different values and can use only the two digits in their union. They therefore occupy both digits between them, so neither digit can occur in another cell of the same house.