XML is a special way of marking up text so you can tell which pieces of text are what parts of a formatted object.

RSS is a special way of using XML to mark up the text of blog posts so it can be read and displayed by other applications. It is still XML and can be read and displayed by any XML reader.

Atom is a particular kind of RSS which has added information some applications can use. It is still RSS and can be read and displayed by any RSS reader.

Think of it like a car. XML is a sedan. Oldsmobile is a particular make of sedan. Oldsmobile 98 is a specific model of sedan. But they're all sedans. Get it?

Caliban said it really.

XML is the language - RSS is an application (just a way of using XML for a particular use) and atom is another way of representing the same data (the creator thought it was better than rss).

Depending on how you create your rss 'feed' that will determine whether it ends up as .rss, .atom or .xml

For most purposes all you need to know is what format your feed is in and what format is preferred by any places you want to submit it.

Andy