That's called Quoting. If you look at the post, to the bottom right hand corner theres a box with "quote" in it. Just click that, and you'll be quoting that text. Also if you want to quote one line, you can edit it. You can also use [ quote] ... [/quote ] ( I put a space in there so it won't show properly.
Check out Tilted Newbies for lots of helpful information/people
