Restriction enzyme help

[Shawn Abigail]

I downloaded a file with a list of restriction enzymes, their sequences
and sources.  I am a little confused over the nomenclature of the sequences.
C, T, A & G are obvious!  R stands for a Purine and Y stands for a
Pyrimidine.  Does N stand for "any nucleotide"?  What about W and S?
Any help is gratefully appreciated.

Shawn Abigail
abigail at ramsey.cs.laurentian.ca