An Identity column is a column (also known as a field) in a database table that is made up of values generated by the database. This is much like an AutoNumber field in Microsoft Access or a sequence in Oracle. Because the concept is so important in database science, many RDBMS systems implement some type of generated key, although each has its own terminology.
An identity column differs from a primary key in that its values are managed by the server and usually cannot be modified. In many cases an identity column is used as a primary key, however this is not always the case.
It is a common misconception that an identity column will enforce uniqueness, however this is not the case. If you want to enforce uniqueness on the column you must include the appropriate constraint too.
In Microsoft SQL Server you have options for both the seed (starting value) and the increment. By default the seed and increment are both 1.
Read more about Identity Column: Code Samples, Related Functions
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