Here's a very rough overview. Buckets are the fare classes, like L, U, T, Y, B, M, etc... each one has different restrictions (as to refundability, upgradability, etc) and falls into different price categories.
On Delta.com you see:
coach (lowest fares)
coach (discounted)
coach (flexible)
first/business
But that simplifies it, there is more to it than it appears at first.

There are also upgrade buckets, the classes (Z) that indicate how many upgradable seats exist on a flight... so if you buy an M ticket, hoping to upgrade, but there is no Z availability, you'll be stuck in back.

See this page for more info.