It's called a Rich Site Summary, or as it's usually known, an RSS feed. The page takes content from PokeBase and restructures it as an XML file. (XML files are very similar to HTML files which is why it may look familiar.) You can then pass on the link to the XML file to an RSS feed reader, which lets you add feeds from different websites and access them all together on one page. It's like a way to keep updated with a bunch of different sites at once, without actually having to access each one.
This page has a list of RSS feed readers that you can use, if you like. There is also this page which will probably explain this better than I did.