Service and Service provider Interface

I am new to java's extensible framework and i just started Service Provider Interface. I am really confused between service and service provider interface. When i read first documentation i fully understood the SPI concept and what i got is service and service provider interface are same concepts. But when i read second documentation it separated service from service provider interface in the introduction section, as it lists 3 elements ,first it defined service then service provider interface and lastly service provider. Now i am really confused between service and service provider interface. Kindly clarify me about service and service provider interface.