OK, I assume you're just working for your own pleasure and not at work ?
An identity column is one that is auto generated. You specify the starting index and the amount to add each time, typically these are 1 and 1, so when you insert data, you can't specify these values, they just auto increment. So, the long term result is, you have an id that uniquely identifies each record, which you cannot ever change or set.
http://www.sqlteam.com/article/understanding-identity-columns[
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