What determines peoples income and why do some people receive much larger rewards than other whose efforts appear to be similar?

In a relatively free capitalist economy, the answer is 'supply and demand' - the degree of effort or time put in is much less relevant. Why does a pop singer or footballer get paid huge sums? Because people buy the tracks or go to the matches.The janitor at the sports ground may put in a lot of time and effort, but his work is not the reason why people attend the matches.