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The First Normal Form: Stipulates that no repeating groups or sets should be stored within a table. These similar groups should be stored in a separate table. To identify each unique column of these tables, a primary key should be established

The Second Normal Form: Data Redundancy should be non-existent. Foreign key constraints should be enforced.

The Third Normal Form: Every column in the table should be related and dependant on the primary key of the table. If this is not possible, the field should be stored in a new table with a new key.

The Fourth Normal Form: This one would probably exists somewhere in dream land - the elimination of independent relationships in a Relational Database.

The Fifth Normal Form: Exists in never-never land - it calls for a perfect segregation of logically related  many-to-many relationships.

So now you know something about relationships (that's what our whole Relational Database thing is about right?). But just keep in mind that as we increase and tighten our relationship enforcements, there would be a little trade off with performance.--George Alexander, A fundamental approach to Database design