1. A project sponsor is the project funding member/owner and has supreme authority on the project. He is the final decision maker on all member of the project. A project sponsor is responsible for developing and maintaining business case.
2. A steering committee is the highest decision-making board of the project organization in project management and consists of the people in charge of the project at the customer's organization, sometimes of representatives of all project stakeholders. Its main task is to monitor the project.
3. A project stakeholder is anyone who directly or indirectly connected to Project.
Hope this above definition help to understand, for more details and example, you can google it. Saving Changes...
Non academic definitions:
1-Project sponsor is the person that will be able to you all resources needed and will take care of the project politically.She /He is is like Jesus Christ for you.
2-Steering Committee are the Apostles. It is composed by people that are very concerned for the success of the project mainly because the solution it is creating.
3-Project Stakeholder is any entity that will be impacted by all related to the project, the process and the result. Saving Changes...