Classic definition wise guilds had a legal monopoly or regulation over a certain trade occupations including the quality and the maximium supply through control of pipelines of licensing and training.

Historically they were a often a feudal "link" of control for administration. They meet with the head haberdasher to discuss carrots and sticks of taxation and licensing so they pay willingly and let them shout about unlicensed competitors instead of trying to chase down every last haberdasher to tax.

Unions are more "if you piss off every affiliated habberdasher in the city they refuse to make hats until their demands are met".

Probably. See the “Seattle Police Officers Guild” which is definitely a union... police aren’t independent contractors, or craftspeople, etc.

A guild is a collection of independent contractors, a union is typically your own employees. Also guild sounds way cooler.

Guilds (I assume) involve more mead and iron working.