what is the difference between "xs" and "xsd" for XML schema?

when reading XML Schema Part 1: strcucture and XML Schema Part 2: Datatypes Second Edition, I did not figure out the difference between using "xs" and "xsd",
thanks for your help.

--howard



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Howard Dong
Cogent Systems.
Phone: (626)799-8090 ext 516
Fax:  (626 )799 8996
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