There are four kinds of corruption, from lowest to highest :
(1) cash bribery. I don’t have to explain what it is.
(2) asset bribery. For example, sell an apartment to government officers at price much lower than market value.
(3) controlling business opportunities. If there are good business opportunities, everyone wants a piece of it. So those who can get the offers or contracts are the ones who make money. If you have family business, being a powerful politician secures the business opportunities for your family business in lot of ways. People will love to give the contracts to your family business compared to other available options. Of course you will return the favors in favor through the political power you have, like giving them state contracts, or paying them higher for the contract.
(4) mutual trust between politicians and businessmen, that is, politicians work for businessmen when they are in office and believe he will get in return after leaving office. Why do you think Clinton has given so many 5 min speeches, each paid tens of thousands of dollars? While Brooksley Born, the one who tried to contain Wall St, was never given such opportunities?