The tone of the memo and any kind of paper depends entirely on the audience. If the audience consists of your boss you need to write an email for professional and office use, the tone and structure should be quite formal. But the tone for some friend may be informal and you may talk about general things. The writings for the professional use should be clear, short emphasizing on the right point. You may write in detail about many things to your informal group but it should be concise and clear using very good language and wording for professional use.