A guild is a group of people who all work in the same trade. For example, bakers would belong to a Bakers guild, and bankers would belong to the Bankers guild, and so on.
Guilds were designed to end competition. If you were a guild member, the guild made sure you didn't have any competition in your area. Plus, the guild was on hand if you needed help. Guilds were also social groups throwing festivals and parties for its members.
To be a member of the guild you had to follow their rules. These included: Letting the guild set the price for all your goods, all members of the guild paid workers the same wage, your work had to be up to guild standards (no shoddy workmanship), and finally you couldn't advertise. This was to give all guild members the same advantages.
There were three level within each guild, Master, Journeyman and Apprentice. A master was someone who was the best at what they did and usually owned their own shop or business. Right below the master was a journeyman. This was someone who was skilled in the job but was not quite up to the level of master. As you learned you moved up until you could prove to the guild that you were good enough to be a master. Finally there were apprentices. An apprentice was someone who was learning the skills needed to work within the guild. An apprentice was usually not paid, but was given room and board.