The ability to modify a scheme definition in one level without affecting a scheme definition in a higher level is called data independence.
There are two kinds:
Physical data independence
The ability to modify the physical scheme without causing application programs to be rewritten
Modifications at this level are usually to improve performance
Logical data … Read more
Pretty good article here on the differences and what they both do. Interesting.
Difference between Google Panda and Google Penguin ...