The forum
Threaded · Public archive · Slow
Question-and-answer boards and long-running discussion sites. Everything is searchable, which means the archive is often more valuable than the live conversation. Reputation accrues through answers rather than posts.
Where to look: search your field's jargon plus the word "forum", and check what your professional body links to.
The chat server
Real time · Ephemeral · High volume
Invite links, channels by topic, conversation that scrolls away. Excellent for immediate help and for reading how experienced people actually think; poor as a permanent record. Time-zone dependent in a way the other shapes are not.
Where to look: the community links on tools you already use, conference websites, and newsletters in your field.
The mailing list
Email-based · Long-form · Durable
Unfashionable, still working. Lists tend to hold the most senior practitioners in technical and academic fields, partly because the format rewards writing carefully and punishes posting for visibility.
Where to look: university department pages, standards bodies, and the footers of specialist software projects.
The association
Membership · Credentialed · Formal
Professional bodies, guilds, chambers and licensing organisations. Usually paid, usually regional, and frequently the only venue where the people who set standards in your field are all in one place. Often includes local chapters that meet in person.
Where to look: the body that issues or recognises credentials in your field, and its regional branches.