Looking for SPI documentation I'm trying to use an Arduino as a slave and was searching for examples which I found many. But apart from the examples, I haven't found any documentation explaining many thing that show up in those examples.

For example, I'd like a document that would state what things like the following are:
-SPCR  
-SPE 
-SPDR 
-ISR 
-SPI_STC_vect

I'm asking this, because I don't and can't just copy an example and hope that it'll work, I really need to know what I'm doing.

Thank you, 
João
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.