Monetary Investments.

Poor Man: Works all day and spends his whole paycheck as soon as he receives it on frivolous items or vices.

Well-to-do Man: Works all day, spends his money frugally on essential items for his family and himself. Saves money in a bank account.

Rich Man: Works all day knowing someday he will never have to work again. Brutally tight with finances. Meticulous on how much he spends and on what. Not cheap and purchases quality items. Saves minimum 10% from every paycheck. Uses these savings to invest in a multitude of different businesses and investment vehicles. These businesses he buys or creates work by themselves to create money for him. Same with investments in stocks, bonds, mutual funds, insurance premiums, equipment rentals, etc etc. These investments make him money 24/7 all day every day for the rest of his life thereby adding more and more money to his growing pile. When rich man retires he lives off the interest ONLY of all his investments never touching the principle amounts if he can.