First, open up the PDF file in Preview (Acrobat might work too). Then select Print, and under Pages choose the range of pages you need to extract. Then click Preview. Those pages will open up in a new window and you can select Save As from the file menu to save the extracted portion.
[Editor's note: I'd often wondered why it might be useful to be able to select "Preview" while already running Preview ... and now I know why it might be useful!]

