What is a PDF again??

I had a bit of a look on the internet about PDF's as I have heard of them but have not used them myself. I was trying to upload my instructions as PDF to my webpage and I couldnt quite work it out. :-( I spoke to one of the girls in my workshop that uses PDF's regulary at her work and she told me how to create it through the 'save as' button in microsoft word. Well that was easy !