Welcome to the forums!
Within the New Horizons forums, the
general discussion board is for general chat about the game, and there's also a thread for sharing in-game screenshots.
The Airport is an online playing board, if you're looking to visit other islands or have others visit you. People post in there to invite others to their island, most often for tours of their island or if there is a special visitor, like Celeste.
Nook's Cranny is a trading board, for buying, trading and selling in-game items. A lot of trading happens using a forum currency, which we call "TBT bells". You earn TBT bells by posting on the forum (The Bell Tree is nicknamed TBT). Here's a guide on how TBT bells work:
Bell System Beginner Guide!
Turnip Stalk Exchange is for turnip prices, and the New Neighbor Network is for trading or giving away villagers.
There are non-ACNH sections, e.g. Brewster's Café (for non-AC chat), The Basement (for forum games) and The Museum (a place to share art/creative endeavours)
I hope that helps a little bit, but if you have any questions, please don't hesitate to ask!
I hope you enjoy it here!