What is SCD.?

SCD: Slowly changed dimension

The SCD how to Capture the change data with respect to the time.

For ex: In an organization emp sal is chaged 
        according to his perfomence after chaned sal we kept in 
        datawarehouse. 

(Datawarehouse maintain a Historical data )
there are 3 types of SCDs.
Type1: It will maintain curren data only. It does't 
maintain historical data.
Type2: It maintain current and historical data.
Type3: It maintain current and previous data it mease 
Parcial data only maintain.
So generelly datawarehouser is maintain Historical data 
hence Type 2 is useful