Batcache uses Memcached to store and serve rendered pages.
Batcache can be used anywhere Memcached is available. WP-Super-Cache is preferred for most blogs. If you have more than one web server, try Batcache.
Batcache was written to help WordPress.com cope with the massive and prolonged traffic spike on Gizmodo's live blog during Apple events. Live blogs were famous for failing under the load of traffic. Gizmodo's live blog stays up because of Batcache.
Actually all of WordPress.com stays up during Apple events because of Batcache. The traffic is twice the average during Apple events. But the web servers and databases barely feel the difference.
Batcache was named "supercache" when it was written. (It's still called that on WordPress.com.) A few months later, while "supercache" was still private, Donncha released the WP-Super-Cache plugin. It wouldn't be fun to dispute the name or create confusion for users so a name change seemed best. The move from "Super" to "Bat" was inspired by comic book heroes. It has nothing to do with the fact that the author's city is home to the world's largest urban bat colony.