Discworld concepts help
Guilds exist to add depth to the game. You have the burly, fighter type guilds which exist solely so that their members are the best people on the entire Disc at going "Ugh!". Then you have the frail wizard type guilds who specialise in taking a small amount of magic and turning it into the swift demise of an opponent. You also have a wide variety of other classes into which you can take your life.
The way the guilds aid you is in cheap advancement of skills. For information on how skills work read the skills concept information. Guilds provide cheap training in skills that are appropriate for that guild. For example, a warrior can get cheap training in all fighting skills. Guilds are also stingy about providing training in non appropriate skills. And if they do provide training they do not allow you to train very far in skills.
You will find that to form a successful party, you will need members from various guilds. A warrior for maintaining weapons and armour and splatting little monsters. Oh, and drinking copious quantities of alcohol. You then have the wizards who exist for doing lots of damage and providing the party with useful spells. You have a priest for healing, and protection spells. You can take a thief with you to help you with the damn locks and spying. There is a witches guild whose members can do very interesting things to monsters through the use of the occult.
The guilds exist to make the mud more fun. They make your character more individual. If you feel your guild is hard done by because another is clearly superior, tell the administrators so something can be done. Don't just join the better guild. A mud filled with members of a single guild would be a boring place to visit.
You can wander around as a non member of a guild, aka an Adventurer if you so wish (during which time you can only advance your skills by teaching yourself or having others teach you). This allows you to get a feel for the game and ask around before you make a decision on a guild that you may be unhappy with.
Warriors, priests, thieves, wizards, assassins and witches.
Guilds in Testing:
This is secret. I could tell you, but then I'd have to kill you.