Well, for one, the 777 is bigger than the 767. It is a medium-to-long-haul aircraft and can be configured to reach very far distances. The upcoming B777-200LR "Worldliner" is gonna be the aircraft will the longest range (16,417km). That's more than half the earth's circumference. So you can really fly to any city in the world non-stop. That's the Boeing tagline anyways.
Size wise, the shortest 767, the -200 variant, is 155 ft long, whereas the 777-300 is 242 ft long. The 767 really cannot compare.
The 777 evolved from the 767 design and there are many interchangeable parts btw them. It's tough for a direct comparison btw the two because there are a few variants, each with its own pros and cons:-
B767-200
B767-200ER
B767-300
B767-300ER
B767-400
B777-200
B777-200ER
B777-300
B777-300ER
B777-200LR
You can find a quick comparison of the main models here...
http://www.airliners.net/info/