Postscript is actually a computer language. Really. It is specialized as a page description language, but it is code that executes within a postscript interpreter that exists within printers, applications, or RIP engines that license the proprietary Postscript interpreter from Adobe... or which emulate the Adobe Postscript interpreter, like Ghostscript.
EPS files are "encapsulated postscript files" which contain the actual code the describes a page. It may include embedded fonts or bitmaps.
PDF, in many ways, is just an updated postscript language based file format that is optimized for document delivery and includes extensions, like tables of contents, annotations and other features that add value for document reading. There's a lot of technology in common with Postscript, and one could probably say that PDF is a mostly a repackaging and re-marketing of Postscript technology to apply it to desktop document viewing, as well as print... which was the problem domain that Postscript was developed to address.
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