If you want to reply to a thread and reference text in a post, when replying, press the "Quote" button on the post itself, instead of "Post Reply":

Attachment 9971

This will bring you to the reply window, but now you will see the following already filled out in the window.

This is some quoted text!!

When you press "Submit Reply", the post will show up in a quote box, like this:
Quote Originally Posted by AaronBBrown
This is some quoted text!!
You can also add the quote tags around some text in your reply by highlighting some text in your post and pressing this button:
Attachment 9973

If you want, you can type the tags yourself, as well. Make sure to remember to include the closing [/QUOTE] tag!

Also, keep in mind that, when replying to a thread, the entire thread is visible on the reply page if you just scroll down. This makes it easy to quote multiple people in one post by copy/pasting the text you want and just adding their user name, ala [QUOTE=Username].