U can relate this with Global Sheet and Local Sheet of 
Datatable 

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 respository. 

In per-action instructs QTP to refer to the external action 
repository whereas in shared instructs QTP to use test 
shared object repository for this action.

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

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

Shared Repository is like a golbal file where we can use by 
multiple tests .
Where as per action is like a local file we can use it in 
single test only.