NORMALIZATION

The rules of normalization, the "normal forms"(NF)
are used with the Relational Model in order to create 
and preserve "data integrity".  There is a "cascading" assumption.
That is, if tables are in 3NF, then they are, by definition,
also in 2NF (and if in 2NF, they are in 1NF)

1NF:  No repeating attributes.  That is, a table must not include
an indefinite number of duplicate column headings.  For example,
in the CUSTOMER table, there cannot be an indefinite number of
columns called ORDER_NO.

2NF:  Every attribute must be fully functionally dependent on the
entire primary key.  [...the key, the whole key, and nothing but
the key, so help you Codd.] This rule applies only to tables with 
combined, or concatenated, keys.  A violation occurs when an attribute 
is functionally dependent on only one part of the concatenated key.

3NF:  No intransitive dependencies.  Non-key attributes must not be
functionally dependent on other non-key attributes.  The dependency of
"city" and "state" on "zip code" are classic cases of intransitive
dependencies.  The solution to violations of 3NF are to build 
another table.