1. Shared repository objects can be used for the entire test where as Per-action repository objects can be used for that particular action only.
2. Shared repository occupies less memory compare to per-action repository.
3. If any change occurs in application we need to make changes only once in shared repository where as per-action repository needs change in every action repository.

OR

Per-action: In qtp we select per-action mode then for each action it creates a object repositary.This type of process is memory wastage process.

Shared repository: In qtp we select shared mode then for all actions it creates one object repository in a test. This not memory wastage process we can this repository in other tests also.

 Hope you will understand

Thanks and Regards

Nitin Patil,Pune