It seems everyone has their own version of the story about Apple stealing the GUI from Xerox. Bill Gates uses the story to discredit Apple's claims that Microsoft violated Apple's intellectual property rights by hacking together its "Mac-like" Windows. Steve Jobs' version of the story paints him as the man who was struck by a vision of the future of computing when shown some WiMP-like widgetry at PARC (a privilege for which he paid in Apple stock worth about one million dollars). It's worth noting that Xerox didn't seem to notice that anything had been "stolen" from them until the Macintosh had been on the market for several years.
In any case, each of these stories is too simplistic to be the whole truth. Even the original Macintosh engineers differ over the origin of key concepts in the interface. However, one thing these latter discussions reveal is that the story of Apple swiping the concept wholesale from Xerox is simply an impossibility. Like the Internet, many of these concepts had been floating around since the 1960s, when neither Apple nor Xerox PARC existed. What's more, further key concepts were dreamt up entirely by Apple, such as "drag and drop", and others seem to have been arrived at independently at several sites.
One of the best places you can read up about all this stuff is at Apple History which reprints an exchange of articles between Bruce Horn and Jeff Raskin, both members of the original Macintosh team (Raskin is more or less "The Father of the Macintosh"). You can read this at http://www.apple-history.com/GUIdiscussion.html - here's a couple of key quotes from Raskin:
"As I said in my history of the Mac Project ...
the Mac was by no means the work of one person, but the combined efforts of
thousands in hundreds of companies large and small. It was not, as many accounts
anachronistically relate, stolen from PARC by Steve Jobs after he saw the Alto running
SmallTalk on a visit."
"...it is perhaps
understandable that people would find it necessary to invent a history that derives the
Mac's genesis from the nearest similar work. The honest intellectual debt the Mac
owes to the work at PARC was not a case of highway robbery.
Oh, and one more point: the mighty Woz was not involved in the Macintosh project.