PROFICIENCIES
Proficiencies are things that your PC can learn. Examples are skills, spells, songs, instruments, weapons, and languages.
Most classes are able to learn certain skills. For example, the fighter class can learns the bash skill while the thief class can learn the backstab skill. In order to learn any particular skill, you must have enough "build points", or skill points. Everytime you advance a level, your character will acquire more build points. If you have any build points available to your character, you must find your guildmaster and use the "practice" command to learn the proficiencies.
Depending upon how hard the skill is to learn, and how intelligent you are, it will take a number of build points to fully master any particular skill. In addition, you will not be able to learn most skills right away. Many skills have a level restriction. For example, you must be at least the fifth level to learn the warrior skill "rescue".
In addition, certain classes may not be able to fully master a certain skill. For example, clerics are only able to learn an extra attack up to 50% ability. Also, there is a 5% penalty for every extra class that you have. For example, a single classed mage can learn the "magic missile" spell up to 95% ability. (Most of this discussion on skills applies to spells as well.) A dual classed mage/warrior can only learn it to 90% ability while a triple classed mage/warrior/thief can only learn it to 85%.
If you would like to see the skills that your class can learn, use the "all" command. Example: "all thief" will show you a list of all the thief proficiencies that you can acquire as well as how good you can get at them and at what levels you get them at.
See profs for a description of the skills command.