If you watch enough bowling, you've seen and heard people talk about buckets. What is a bucket? A specific spare leave, involving four pins in the shape of a diamond. Most bowlers distinguish between a right-handed bucket and a left-handed bucket. That is, for righties, the bucket is the 2-4-5-8. For lefties, the bucket is the 3-5-6-9.
The 1-2-3-5 is the only other leave in the same shape, but is not commonly referred to as a bucket.
As with any leave, the goal is to pick up the spare (or better, strike on the first shot and avoid having to throw at a bucket). Most bowlers will throw at a bucket using their normal hook shots, adjusting their positioning to have the ball hit the bucket in the same way they attempt to hit the pocket on their first shots.