Looking for SPI documentation
You are confusing SPI, a de facto standard, with the Arduino/Atmel hardware support for SPI.

SPCR, SPE and SPDR are registers of the Atmel MCU, part of the hardware support. 

ISR is Interrupt Service Routine and SPI_STC_vect is the vector that point to that code. That is where the data received from the cable is read finally.