Abby - ebay has what's known as an API (Application Program Interface) that allows places like auctiva to communicate with ebay's server.
in the past, ebay used XML for this communication. In typical ebay fashion, they've changed things around and will no longer be supporting XML. this means that places like auctiva have to rewrite the code their programs use for communicating with ebay.
Auctiva has it all taken care of, so no worries
Some of the other places dont. it also means that any of the older desktop tools that you may have downloaded for launching auctions from your computer will be out of date.
Like Suthernjewl said, not really anything for you to worry about