No aspect of life in Waterdeep goes untouched by at least one of its more than forty guilds. Virtually every profession has an associated guild, and there's hardly a citizen of the city who doesn't belong to one or more guilds, or doesn't work for someone who does. As a visitor to Waterdeep, you need to know this, lest you run afoul of “Guild Law.” Guild Law isn't technically in the legal code of Waterdeep, but guilds are mentioned in the oldest surviving legal documents-penned by Ahghairon himself-and the rules of Guild Law are respected by wise city folk.
Guilds take their laws seriously, as do members of the City Watch and the magisters. If you flout a guild's traditions, you can expect not only public scorn but also a visit from enforcers of the law. In addition, many guilds have their own codes of accusation, trial, and punishment, such as:
Many guilds have codes that entwine each other, complicating matters even more for the outsider. In Neverwinter, if you want to construct a building, you simply purchase the land and hire workers to build it. In Waterdeep, the Surveyors', Map-, and Chart-makers' Guild must first be consulted upon designation of the plat, then brought in to draw or approve the construction plan. The Cellarers' and Plumbers' Guild must then clear and prepare the site, only after which will you be able to hire members of the Carpenters', Roofers', and Plaisterers' Guild to erect the structure.
Moreover, the work will not be complete until members of the Guild of Fine Carvers and the Guild of Stonecutters, Masons, Potters, and Tile-makers design and craft any decorative elements of wood, stone, or ceramics, and after the Most Careful Order of Skilled Smiths and Metalforgers has manufactured and installed any door hinges. If the building is to be connected to the sewers or a city water supply, the Cellarers' and Plumbers' Guild must be called upon again to do that work. Want glazed windows installed? For that, you need to hire members of the Guild of Glassblowers, Glaziers, and Spectacle-makers.
If you do business in the city as anything other than a purchaser of goods and services, I strongly advise you to seek out a local solicitor and pay to be guided through the process. No guild of solicitors exists, so be sure your choice comes highly recommended by individuals you can trust. To learn the peculiarities of any guild's rules, consult someone on duty at the guild headquarters or ask a senior guild member.
All that said, working at a guild-related profession without being a member of that guild isn't illegal. Guild members have no lawful recourse to interfere in the business of someone who chooses to not join the organization. But if you practice a trade or operate a business without becoming a member of the appropriate guild, word spreads, and you'll find that your coin isn't good for purchasing the goods or services of anyone who is a guild member. Since that group includes virtually everyone who sells the necessities of life or offers shelter for a fee, the benefits of joining a guild swiftly become apparent to those who procrastinate in this regard.
Any characters with the guild artisan background are assumed to have free membership in one Waterdeep guild of their choice. Arcane spellcasters living in the city are required by law to register with the Watchful Order of Magists and Protectors, so that they can be called on to defend the city with their magic in times of need.
Waterdeep's most prominent guilds appear on the Guilds of Waterdeep list. By law, there are no active thieves' guilds in Waterdeep, and the Xanathar Guild isn't an actual guild but a faction (see “Factions in Waterdeep,”).
To join a guild, one must possess a background, a proficiency, or a status that the guild values. For example, a character with the sailor background is welcome to join the Master Mariners' Guild. One can practice a profession and not be a guild member, but such autonomy carries a cost. The guilds do everything they can to entice new members to join and pay for membership, and they go out of their way to hinder independent business owners, up to and including driving them out of business. A baker who refuses to join the Bakers' Guild might find her flour supply cut off, while a wizard who refuses to join the Watchful Order of Magists and Protectors might be denied access to sages, libraries, and other useful resources.
Guilds charge members monthly or yearly dues that vary. (DM Note: For simplicity, all guild will charge 1 gp per month or 10 gp for a year's dues paid in advance.) A guild can expel a member for any number of reasons, not the least of which is failing to pay dues.
For more details, have a look at Guilds of Waterdeep