Relational tables have six properties:
Values Are Atomic
This property implies that columns in a relational table are not repeating group or arrays. The key benefit of the one value property is that it simplifies data manipulation logic. Such tables are referred to as being in the “first normal form” (1NF).
Column Values Are of the Same Kind
In relational terms this means that all values in a column come from the same domain. A domain is a set of values which a column may have. This property simplifies data access because developers and users can be certain of the type of data contained in a given column. It also simplifies data validation. Because all values are from the same domain, the domain can be defined and enforced with the Data Definition Language (DDL) of the database software.
Each Row is Unique
This property ensures that no two rows in a relational table are identical; there is at least one column, or set of columns, the values of which uniquely identify each row in the table. Such columns are called primary keys. This property guarantees that every row in a relational table is meaningful and that a specific row can be identified by specifying the primary key value.
The Sequence of Columns is Insignificant
This property states that the ordering of the columns in the relational table has no meaning. Columns can be retrieved in any order and in various sequences. The benefit of this property is that it enables many users to share the same table without concern of how the table is organized. It also permits the physical structure of the database to change without affecting the relational tables.
The Sequence of Rows is Insignificant
This property is analogous the one above but applies to rows instead of columns. The main benefit is that the rows of a relational table can be retrieved in different order and sequences. Adding information to a relational table is simplified and does not affect existing queries.
Each Column Has a Unique Name
Because the sequence of columns is insignificant, columns must be referenced by name and not by position. A column name need not be unique within an entire database but only within the table to which it belongs.
Reference : Pinal Dave (http://blog.SQLAuthority.com)