Arduino AVR good link needed

Projects Programming
1

Hello!
I found out that one can write and read the ATmega2560 registers and IO ports of my Arduino Mega 2560.
for example some timer1 registers:
TCCR1A = 0;
TCCR1B = 0;
TIMSK1 = (1<<TOIE1);
etc.
I know that one can use the register a
nd port names found in the controllers datasheet, so that's ok.
There are other "constants" or reserved words though, like TIMER1_OVF_vect etc.
So my question:
Can somebody give me a link to a site where all these things are explained and the reserved words and/or constants are listed.
Well, one link is this: avr-libc: AVR Libc , but I would like to see things explained more illustratively.

2

Datasheet...