Income, also called revenue or sales, is the money you get when you sell something. It's good. And if everyone paid in cash at the time they bought or sold it would be easy to calculate your profit by looking at the cash people paid you minus the cash you paid other people and voila. But some people pay all at once in cash, some people pay over time, some pay in advance (like a last month's rent deposit), and some people skip town without paying. If your customers are paying you over time, but you have to pay your vendors when they deliver, you still could be making a profit, but run out of cash (see the need for financing capital). Oh yeah, and remember that there is a time value of money. That $10,000 you are paying on your student loan isn't really worth $10,000. That's why you have to pay interest. That's why accountants are paid a lot to figure out how much money is really coming in.
Sometimes even the cash that comes in shouldn't count immediately as income. Some companies have a 'revenue recognition' problem. They receive payment for work to be done in the future. Is it fair to count as income cash that you've received for work you haven't done yet? Ask PricewaterhouseCoopers.
So a relatively simple concept like how much money you have made is complicated by when you get paid and what you are getting paid for.
