What are "Levels"; and how do I gain them?
Levels are thresholds of experience and ability, represented in a quantified way. Higher levels are better.
For example, many Skills are available only to characters at a certain Level. As you gain Levels, more capabilities become available.
Your character rises to new Levels after accumulating some necessary quantity of Experience. Your character automatically accumulates Experience as you interact with the TriadCity World. In the GUI, the Experience tab will show you how much you've got, and how much more you need in order to reach the next Level. With a MUD client, use the Status command to see where you're at.
Once you've accumulated the necessary Experience, there's a little ritual you have to perform in order to rise to the next Level. Go to the Temple of the King on Sanctuary Island, and bow to the statue there. A message announcing your accomplishment will be automatically broadcast to everyone listening to the Level channel.
Note it's in your interest to level as soon as you've accumulated the required Experience points. The longer you wait, the more additional Experience will be required for you to reach the subsequent level. Level early, level often.
Note there are no limits to the Levels you can achieve.