This is an excellent question, if somewhat brief. Normalization is the process of modifying a data model or database design so that the resulting entities or tables are in normal form.
Here are some resources:
What is normalization?
A definition by Whatis.com
Wikipedia entry for normalization (includes good links to other resources)
Part of Introduction to Data Modeling by the University of Texas
Please define Fourth Normal Form and Fifth Normal Form
An Expert Knowledgebase answer by Craig Mullins
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