Tensore79:
My problem is that I can't understand where these "SPCR - SPDR - SPSR - ISR(TIMER2_OVF_vetc)" are defined. . .
They're defined in Arduino.h (which is always included in your
program whether you added it or not)
They're NOT program variables, they refer to hardware registers on the CPU.
When you type "SPCR=5" you're putting the value 5 into the CPU's SPI control register.
The datasheet for the chip has all the details of what that will actually do - it might tell the SPI unit to send a byte of data (for example).