As you rise to higher levels, you will gain more powerful spells and skills. Each class has their own unique spell and skill abilities. Some races also have special abilities too. You can view your character's list by typing PRACTICE. To improve your knowledge of a spell or skill, you must simply use that ability. You can also improve your knowledge of some spells and skills by finding a book or scroll on the subject to STUDY.
Magic Users and Clerics can CAST their spells. Alchemists must use their special ability to CONJURE and RECORD to create potions and scrolls from their spells. Warriors and Thieves can use most of their skills by just typing the command itself, such as KICK or SNEAK.