PDF stands for "Portable Document Format". It was invented by Adobe; you can find more information about it at http://www.adobe.com/products/acrobat/adobepdf.html .
It's an excellent format for sharing documents, because, unlike, for example, HTML, it preserves formatting exactly and it can be self contained: you can embed all the fonts for a document within the PDF. It has some disadvantages compared with HTML: it doesn't reflow and is slower to search through, for example. But when you need or want a document's formatting to be preserved in detail, PDF and Postscript are the only open specifications you can turn to, aside from just using images.