Much the same as the Job concept, you could grow and advance in these guilds. Blizzard would make the quests and perhaps release one a week.
(One of the differences between job, is this would have a lot to do with your social standing, who talks to you, who listens to you, who sells to you, ect. ect. This would be MUCH more important then a job, although wouldn't give you money, would perhaps give you Stat points, Skill points or Mastery ponts)
Guilds would vary in size and attitude. Some might be very formal, traditional and upper-class, while others may be laid back, slack and relaxed about everything.
Guilds would all be rivals, so while you may join more then one, it may not be good for your guild advancements.
Something I can't describe without example is this;
AAA Mage Guild may have a guild in Tristram. Since they are a very famous and LARGE guild by nature (Snotty nosed at that) they have AAA Mage Guilds in every city all over the world (Not towns and villages, but may every major city or capital) so you can always keep in touch.
At the same time;
Billy the jester's Theives guild may only have the one place under the library in the capital of ___land.
While you can indeed cross breed your character to be multiclassed and INDEED infact multiguild, you may one day have to kill one of your close assosiates or something alike against your own guild.