What you’ll do is select the text you’d like to email (or send through Messages, or tweet about, or add to Notes, etc.) and then right- or Control-click on it. When you do so, a contextual menu will appear, and one of the available options is “Share.” If you hover over that, you’ll see your choices.
Pick “Mail,” and the text you selected will be inserted right into the body of an email, ready for you to pass it along. That sure does make things faster! And if you select “Messages” or a few of the other options, you’ll instead get a little box overlay for you to compose and edit as you see fit.
This works in quite a few places around the operating system, including Safari and Mail, so if you need to forward only a bit of a message to someone else, for example, you can do so. That’s awesome. I just love step-skipping!