/2 or /trade is the trade chat channel. Used for well... trading/buying/selling of junk.
Spamier than the auction house (AH) but generally there's going to be more "rare" items in /trade
This channel goes across all haven instances. Disabled by default now.
Type /join 2 (or /join trade) to enter, /2 whatyouwanttosay to send "whatyouwanttosay" to the channel, /leave 2 to leave it.

/1 or /z or /zone is the zone chat channel
Used to send a message to all players in the same haven instance
Generally frowned upon using this channel for selling/buying/trading things
Since the inclusion of /2 i haven't seen much usage of this channel.
Good for making/finding partys for a specific run. You are in this channel by default.
/1 whatyouwanttosay to send "whatyouwanttosay" to the channel, /leave 1 to leave, /join 1 to rejoin channel.