"Send Message" transmits your email message to the intended recipient(s). Make sure you do want to send your message as this action is final and cannot be reversed.
"Save Draft" places an unfinished message in a folder called "Drafts" accessible through the folder drop-down box at the top-right of your screen. You can then finish the message later by switching to the "Drafts" folder, clicking on the saved message to open it, and then clicking on "Resume". Be sure to put a subject on the email before you save the draft so that you can easily identify which message you want within the Drafts folder.
Depending on your installation, you can change in your preferences whether you are returned to the last folder you were viewing or back to the same message after saving a message as a draft."Cancel Message" drops the message in progress and simply returns you to the last folder you were viewing.