They were shite but he's been whipping them in lately, thankfully.
Anyway, most of these sites us Opta and their definitions are as follows:
Chances Created
Assists plus Key passes.
Goal Assist
The final pass or pass-cum-shot leading to the recipient of the ball scoring a goal.
Key Pass
The final pass or pass-cum-shot leading to the recipient of the ball having an attempt at goal without scoring.
Which means that any old header doesn't count, it has to be an obvious attempt at goal.