The guild system is rather complex at the time of this writing we have 104 guilds.
The basic idea is this, for every normal level you advance you are given what we call 1 'open guild level' this means that you can then advance in the guild of your choice. For example if I advance 2 levels I would now have 2 'open guild levels' which means I could go to my guild of choice and advance twice (assuming I meet the other gold/exp requirements).
The guilds themselves are broken into 5 levels in order those levels are:
There are 5 alpha type guilds. Alpha guilds are the lowest guilds on the guild tree and basically define what type of character your going to become. The 5 alpha guilds as of February 1st 2000 are:
Beta guilds require the membership and fullfillment of one alpha-level guild. Beta guilds are slightly stronger guilds than the alpha guilds.
The next branch on the guild tree structure. A player needs to fullfill one or more beta type guilds to join a particular gamma guild.
Level 4 on the guild tree. Guilds at this level are extremely powerful. Spells and skills are approximately twice as strong as gamma guild spells and skills.
The top level on the guild tree. If you max out one of these guilds, you will have an ultimate character.