Arm straight up: offside

Arm towards goal line: goal kick

Arm towards corner: corner kick

Arm in one direction or the other indicates throw in direction (if it was a close call -- often there isn't any chance for dispute and the team knows what's what and throws it in within a second or two if there is a hurry).

There is a "play on" type arm wave (usually accompanied by a verbal "play on" or something) to indicate "yes I saw him fall, no I don't think it's a foul, keep playing".

The line referees have flags and have similar signals. The only real difference is for a foul they hold the flag straight up and wave it a little. For offside they hold it straight up and stand still to mark where the offside occurred. The line referee should always be located at the exact line that marks offside. Of course pro players are faster than referees, but to get past level 7 refereeing (starts at level 9, kids games, level 7 is probably high school games, state level, etc) you must pass fitness exams as well as paper exams. Level 2 is FIFA assistant referee, so they must all be in quite good shape.