Select Menu > Chat.
You can exchange instant messages with your friends. Chat is a network service. If you do not have a chat account, you can create a Nokia account, and use Nokia's Ovi Chat.
You can leave the Chat application running in the background while you use other features of your phone, and still be notified of new instant messages.
Using services or downloading content may cause transfer of large amounts of data, which may result in data traffic costs.