The quote feature uses simple open/close quote tags in the body of your post.  Please note that, in the following explanations, the example quote tags are enclosed with double quote marks (") and a space is inserted after the first "[".  This is necessary so the system doesn't turn them into actual quotes in this post.  You should type the quote tags without the " marks and without spaces (except as noted below).

Use the "Post reply" button (top and bottom of the page) if you simply want to add a comment to the thread that isn't related to another existing post.  If you wish to reply to a specific post, and have your post show the "(Reply to #nn)" link, use the "Reply" button of the post itself.

Think of the + Rep / - Rep buttons as being similar to like/unlike buttons on other sites.  To rep a member's post, click the + Rep button.  After you've repped a post, the button will then show - Rep, allowing you to remove a rep if you clicked the button by mistake.