You would use Quote if you intend to include parts of the post you're replying to in your answer. Note that you should edit that quote down to the relevant part; quoting an entire post is generally unhelpful. Otherwise, it's the same as Post Reply.
rajesh yedula wrote:When to use QUOTE and when to use POSTREPLY.
Ulf clicked the "post reply" button, whereas I clicked the quote button, allowing me to indicate what part of your message I am specifically responding to. While Ulf's answer was responding to your entire post, I am responding to the specific question of when to use each, and by quoting you, it should be obvious that I am doing so.