Service Entry, Initial Operating Capability etc?

Wondering if someone could clearly something up for me (and guessing some other people may be confused).
But whenever an air force gets a new aircraft, it is often some time before they get ‘initial operating capability’.
Even the KC-30A for the RAAF, they are receiving their first one next month, but don’t expect to get initial operating capability til ‘late 2012’. What takes the time? What is the next step ‘full operating capability’? Is that due to numbers or aircraft specific?