It's a fairly complex area - for example some examples:-
1... Prize money won by a professional in the course of his profession is taxable. This would apply to the case of a professional golfer, temmis player, etc. but could also apply to someone who competes professionally in artistic merit competition - e.g. professional dancer, ice skater, etc.
2... Prize money won from a source outside the country e.g. a foreign lottery is taxable
3... Prize money won by a professional gambler, including prize money won in games of chance is taxable i.e. he/she is taxable on the net annual income from this source