Revenue - Expenses = Net Income
Net Income - Capital = Profit
Revenue is all money taken in (from services rendered, mostly.)
Expenses can be a wide range of things such as investments, supplies, equipment, property... the costs of any assets obtained and the cost of doing business...
Capital is the original amount put up to start the business.
As Heisenburg said, until the Net Income has become bigger than the capital, you haven't really made any money, because capital is money spent from the beginning and still must be accounted for.