A guild is an organization that negotiates for certain minimums in contracts for those in a particular industry. They set minimum rates and minimum benefits for anyone who gets hired in that particular field. Members of a guild have to pay a certain amount of dues to fund it, but members get other benefits such as being able to purchase health insurance through the guild and also as a form of social networking.
Now, people are not forced to join a guild or abide by the minimums put froth by guilds. However, there are consequences for doing so. For example, anyone who works for a businesses who pays lower than the standard minimum set by the guild may find themselves ostracized by guild members until they pay fines to the guild. This is mostly a problem for those starting out in a certain industry, though, as those who have acquired the skills and expertise of their chosen occupation usually works for above guild minimum pay anyways.
A good example of this are the many guilds in the entertainment industry, such as the Screen Actors Guild or the Directors Guild of America. These guilds don't set pay for actors and directors, unlike more labor-oriented unions. Instead, they focus on contract minimums (such as minimum pay and basic benefits) and basic working conditions.
What this does is protect those involved in the guild but gives better leeway for market forces.
So, under a guild, workers would be able to organize to demand certain minimum protections (which gives the benefits of unionization) but would also give individuals and the company they work for more room to negotiate individual contracts (which give the benefits of individual rewards).