What are the primary differences between an HP laser printer PCL
driver and a postscript driver?

Hello Skip9801,

PCL - Printer Control Language
  ://www.google.com/search?hl=en&lr=&oi=defmore&q=define:PCL
[though note the first link in this list is broken, the others I checked are OK]

There are good summaries of the differences between PCL and Postscript at
  http://www.cof.orst.edu/net/howto/addprinter/index.php#PSvsPCL
describing the different driver types in general terms. Note the term
"complex features" may include something as simple as font scaling
which cause Excel spreadsheets to print incorrectly (or not at all)
when printing at different zoom scales.
  http://www.prepressure.com/ps/whatis/PSalternatives.htm
This looks at PCL (and several other page description languages) as
alternatives to postscript. There used to be a number of printer
languages, basically one per vendor, but postscript was designed to be
portable across a number of printers.

There are a number of biased definitions as well at vendor sites [sigh].

Basically, both PCL and Postscript are page description languages. PCL
was developed by HP. Postscript which was developed by Adobe.
Postscript is very comprehensive and can be used as a programming
language such as described at
  http://www.marypat.org/ifs.html
which has several fractal pattern generators as postscript files. PCL
is much simpler, originally developed to print text but extended in
several versions to handle graphics, outline fonts, and other
capabilities.

So, a PCL printer driver will render your document as a stream of PCL
data and send it to your printer. In a similar manner, a Postscript
printer driver will generate a postscript program and send it to your
printer. If the source document is relatively simple, you should get
basically the same output on the printer. If not, there will be
generally some differences in alignment, pagination, and other
features. You may also have a document that can be printed in
Postscript but not in PCL (e.g., the fractal examples).

To recap:
 - a PCL driver will generate PCL data for the printer
 - a Postscript driver will generate a postscript program for the printer
 - both will attempt to print an accurate representation of your
document but may have minor differences
 - as noted in one of the references, a PCL driver may do more work on
the computer and less on the printer. The postscript driver will do
more work on the printer.

If you are using a modern printer, you likely get good support for
both PCL and Postscript and the results for both should be similar.

Search phrases used:
  define:PCL
  define:postscript
  compare PCL postscript
  features PCL postscript

If this answer is somehow incomplete or unclear, please make a
clarification request.

  --Maniac

Great answer - complete, full of information, includes links for more details!