The level command is used to raise a level. This is because the training system keeps track of how many times you practice per level. When you issue the level command you raise a level, gain your ability and skill points, and can start a new (cheap) training session.
Experience is usually gained by killing monsters. The experience you gain is relative to your level compared to the monsters level.
Remember that you may only practice a certain amount of skills/spells/ abilities per level, thus it is to your advantage to only type level once before practicing.
You gain no experience when killing players.When cast upon an object, player, etc., the contents of that object will be revealed.