Public private partnership or PPP is a funding model which exists between public that is government and private parties.
These type of partnership is taken only when there is a need for a project which is dedicated for public use like airport, bridge etc.
The public part in this partnership is represented by givernment and the private part can be any company which is privately owned or public corporation.
PPP is a term that can be given to a short term model which do not involve investment to a long term which do involve investment.
This type of partnership is useful for project that requires a highly skill workers and a large amount of cash for its working .