Three types of DSO in BI 7.0

Hi,

I went through the sap help documents related to the three types of DSO.

1. Standard

2. Direct Update

3. Write Optimized

Still not clearly understood in which case we use what kind of DSO's.  Could any one explain me that in your own words.

I am new to BI, your explanation is appreciated.

What is this ADP process that is used using the Direct Update DSO.........Any docuements or links or info on it?

Thanks,