Help: Messages
The Message section contains three folders in the sidebar, 'Inbox', for all of your incoming messages, 'Sent' for all of your sent messages, and 'Drafts' for all of the messages that you have been working on and wanted to save to send later. The messages in the 'Inbox' can be filtered by clicking on the drop down menu in the top right and selecting 'All messages', 'Unread messages', and 'Invitations'. 'Invitations' refers to invitations to be added as a contact or to join a project / group. You can select the check box next to an item and click on 'Delete selected messages' to quickly remove messages you don't want to read.
When you click on a message to read it, you will have the option to 'Reply', 'Reply to all', 'Forward', or 'Delete' the message. Sending a message to someone (or replying / forwarding) will enable you to send the message directly to contacts (To:), copy it to someone (CC:) or blindly copy it to someone (BCC:) - i.e. not let the people in the To: and CC: fields know that you are sending it to the BCC: people. You can enter a subject and a message body and hit 'Save as draft' to save it to your 'Drafts' folder or 'Send message' to send it to the recipients. If you want to discard it you can click on 'Discard' or simply click on another link to go elsewhere in the site.