Communication ingame is split into 2 distinct areas (see below) Live Chat, via an ingame chat window and a Mailbox system.
Live Chat allows realtime chat between players when logged into the game. A window will popup displaying a basic chat window where you can chat to everyone who also has that window open.
There are various channels or 'rooms' within the window where players can chat more privately. For example if you are a member of a Faction, all members logged into chat who are in the same Faction can chat there in private. You will also note Parish and Capital channels too.
Visit the main Live Chat page here.
The Mail System is one alot of people prefer, even though there are pro's and con's to it.
Players can effectively 'email' other players wether the recipient is offline or not, much like a normal mail system. Players generally use this for offline mass chat between Faction members and/or allies (where everyone can see the entire chat, instead of having to be brought up to speed in live chat)
Visit the main Mail System page here.