When your character's hit points fall below 0, your character will die.
You will then appear as a ghost - All your unblessed and uninsured items will remain on your corpse and you will be unable to interact with the physical world. You may, however, walk though closed doors, use Moongates, and certain other teleportation pads.
In this form you will be invisible to other players unless in combat mode (which, as a ghost, is entered by attempting to speak). Other players will not be able to understand the meaning of your incorporeal moaning unless you (or they) have some experience as a medium. You can get around this limitation by using party and/or guild chat to communicate.
You will remain in this form until you find a way to be resurrected.
Recovery of Items
You may want to recover some of the items from your corpse. There is a time limit of 7 minutes claimed on Stratics whereby your corpse items will decay beyond recovery. If you return to your corpse in time it will state your name on the corpse and have the likeness of your clothing, otherwise it will appear as bones and the bones will be purged at some future time.