Gaining levels is achieved by accumulating experience points. Once you reach a certain experience point threshold then you will gain a level. You can see how much experience you require to reach your next level via SCORE and can see the experience requirement for each level by using the LEVELS command. Help on EXPERIENCE gives some hints about gaining experience. New levels will gain you additional skills & spells, more hit points, mana points, movement points and practice sessions. What skills & spells you receive depend upon your class, you can see what you might receive in future levels by using the SKILLS and SPELLS commands. Your hit/mana/movement gain when you level is determined purely by your class and race. To be more specific, it is a random value depending on your class, with a small modifier applied according to your race. There is NO other factors influencing your gains in this area. How many practice sessions you receive depends on your class. But its possible to also receive a bonus practice session if you have particularly high wisdom.