As long as = a condition. As long as I have you, I have all I need. (Father to son), OK, as long as (= on the condition that) you've finished your homework, you can go out with your friends. As far as = a limit / an extent As far as I know, Joe isn't going to come to the party. The fields seem never ending...they stretch as far as the eye can see!

"as long as" means "provided that <some condition is true>" or "for as long as <some condition> remains true". E.g. "I'll take you to see the game on Saturday as long as you tidy your room" or "You will obey my rules as long as you're living under my roof". "as far as" means "to the extent <of some measure>". So, "as far as I know, the game has been cancelled" means "to the best of my knowledge, or to the extent to which my information is accurate, the game has been cancelled". or "I wouldn't trust him as far as I could throw him". You trust him to a lesser degree than the degree to which you could throw him (which is mixing up a measure of trust with a measure of distance, but it is still understood). :)