Bottom right, next to the quote button -> Quick reply button -> Multi-Quote button.
These are mysterious? Clicking on quick reply will take you to the bottom of the page where you can reply to a post like 2 seconds faster than you can reply to a post using the "post reply" button. Not that useful if you ask me, but there it is.
Multi-quote button is for quoting multiple posts at one go. Click on the multi-quote button on which ever posts you want to quote to highlight em, then click "post reply" and the quotes will all show up in your reply.
Top left, next to date of the post, thingy that looks like a small sim card?
If the sim card looking thing is blueish, that means it's an old post.
If it's orange, that means the post was made after you last visited the thread.
Or something like that..