A bail is the deliberate abort: you step off before the trick goes wrong. A slam is the uncontrolled crash you failed to bail out of in time.
Bailing is a skill, not a failure. Step off early enough and run it out and you bank attempts; force every trick to the bitter end and you bank slams. The typical mistake cuts both ways, though: bailing too late on a rail costs you a fall from hip height, while bailing too early on a flatground balance trick costs you the learning — a railstand never lands if you step off at the first wobble instead of making small corrections. Rule of thumb: the higher the obstacle, the sooner you bail; the flatter it is, the longer you stay on. And the caveat that gets skipped — bailing only protects you if you can also fall. Rolling out instead of throwing an arm down has to be drilled in isolation on soft ground, and by the street figures only about 8 percent of skaters wear wrist, elbow or knee pads at all.