What exactly does full frame mean?

Fuld frame dslr is with a sensor the same'is size as 35mm film( small variables Can occour).

The 7d, xxd, xxxd and xxxd series is aps-c size, roughly 1,6 times smaller.

Full frame has larger pixels( lower iso noise).

Aps-c has longer reach at same focal lengh.

Depht of field is also different.

This is the main difference apart from price, and onboard flash.

Thomas