Yorker is a term referred to a delivery that pitches right in the block hole (right where a batsman's feet are placed when he takes his stance).
A yorker is a lethal delivery, which is directed into the batsman's toe and when bowled accurately at a high velocity, more often than not, crashes into the stumps. Apart from being a wicket-taking delivery, it comes in very handy during the slog overs of a One-day match where a batsman is looking to give the ball a tonk.
Waqar Younis among contemporary cricketers is known to have a lethal indipping yorker so also his bowling partner Wasim Akram, while India's Zaheer Khan is also famous for his consistency with yorkers. Most successful fast bowlers possess the yorker in their armoury.