Decay is the 'corruption' of a MystCraft world, and is how a created world ends. The types of decay are creep spread, meteors crashing, exploding ground, and other various inconveniences that prevent the full use of a created world for an extended period of time. The more 'unstable' a MystCraft world is, the faster it will decay. Generally, the more benefit's in the world, such as more ores, villages, etc., the more unstable it will become. As such, once you create an ore rich mining world, it's suggested to start mining right away, and to make sure you have a linked book in your inventory so you can escape before the world decays.
There is a confirmed, untested bug that happens when your world has meteor crashes. It causes your client to lock up then crash. Servers themselves seem to be unaffected and continue operating normally.
This will occur much more frequently, when in the local vicinity of a meteor strike.