Each region has a stock set of unique chat channels. These channels are specific to the region and only operate within the bounds of the region.
Those channels are Looking For Fellowship (LFF), Regional, Trade, Advice, Out of Context (OOC) and Role Playing.
As an example, if you are in Breeland, you will have access to the Breeland versions of the above channels and you and anyone in Breeland can converse with them. When you enter another region (Lonelands, for example), you automatically leave your old chat channels and join the new region's set.
Players can also define their own custom global chat channels. Many servers have user-made Looking For Fellowship (GLFF) that you have to manually opt in to. Be sure to check your home server's message boards here to see if they have one running.
The names are self explanatory if you know what they stand for. Many are just for the region you are in. Such as BreeTown has it's own, BreeLand, The Shire, LoneLands. The only weird one is Moria. It seems to be one big region, and not separate channels for each area.
Regional is for regional. This is often used for RP (Role Playing). If you type /say, the text will appear above your character if you have turned the chat bubbles option to "on" in your options/ UI options. If you have not turned the chat bubbles option on, then things said on the "say" channel will only appear in the chat log. In any case, things said in the "say" channel and emotes will only show up in a much smaller, limited area- generally an area that you can see on your screen.
OOC = Out Of Character. This is where you should ask non-game information, and talk about whatever you want.
LFF = Looking For Fellowship. Here is where you look for people to group with. You will see many LF2 which is looking for 2 more or LFG looking for group, usually someone is not in a group but is looking for someone that already started one.
Trade is for trading. If you have something you Want To Buy (WTB) you are asking for something. Usually you will see WTS or Want to Sell.
Advice is a good place to ask questions.
Depending on your server, there are also GLFF - Global LFF, GOOC, GTrade.