As explained by How Stuff Works, inside every lock is a puzzle that can only be solved by the matching key. The main variation in lock designs is the nature of this puzzle, specifically the number of pins and the order in which they ‘unlock’.
Using a tension wrench as a pick, one inserts the pick into the keyhole and lifts the individual pins one-by-one. The object is to lift each pin pair up as if pushed by the correct key, which is indicated by feeling or hearing a slight click. This happens when the upper pin falls into position on the ledge, which keeps the pin wedged in the housing. Using this method, you move each pin into the correct position until all of the pins are pushed completely into the housing, at which point the plug rotates freely, allowing one to open the lock