PDF is an Acronym and FileExtension for Portable Document Format. Invented by Adobe.
PDF files are good for distributing certain types of documents because:
The PDF format was specified by Adobe, the same company that helped introduced PostScript and Type1 fonts.
You can view PDF documents with AcrobatReader (free but not Free). xpdf(1) and GhostView are Free programs that you can use to view PDF files if you don't want to use Adobe's proprietary closed-source reader. KDE has kghostview and GNOME has gnome-gv, both of which are front-ends to GhostView/ghostscript. GNOME also has the newer gpdf, which is based on xpdf for the backend and does a better job than the gv-derived frontends.
You can generate PDFs by using Adobe's commercial AdobeAcrobat, see our CreatingPDFs page.
Apple's MacOSX uses PDF for rendering images (and the user interface), which is why you can have true transparent windows on their OS.
PDF is arguably a version of PostScript and is certainly a descendant of it in some ways. Calling PDF something other than PostScript II was probably actually a very good idea, since there are already enough different forms of PostScript around.