If you are using Apple's Mail program for all of your email correspondence, you probably know that you can hit REPLY or REPLY All , to quote the entire message, and then you can type in some new text in the area above the quote.
When messages get alot of replying back and forth between people or groups, it can sometimes become a big mess trying to figure out what the original message was about. Next time, Select the pertinent part of the message then click Reply or Reply All.
Now, instead of the whole message being quoted, only the part you selected will be in the new message. Now at the top of the message dialog, it is ready for me to type in my response.