The multi quote button. In news posts it actually says "mulit-quote" if you're using the beta. In the forums (and news without the beta) it's the empty bubble next to the quote button. Click it in the posts you want to quote; the bubble will then have the little quote symbol. You can then press the "post a reply" button at the bottom of the thread (above the quick reply box) or press the regular quote button on the last post you want to quote.
what if some one post a huge comment but you only want to reply a piece of that post?
Click the quote button in the forums to go to the advanced posting thing, then delete what you don't want. In the news, click multi quote on the one post and then 'write a comment' like above, and delete the text you don't want.
Or, in either situation, you can quote with the quick reply box and then edit your post and delete the text you don't want in it.