Cash flow is the actual money coming in and going out of a business — not accounting profit, but real cash. A company can show profit on paper while running out of cash (revenue is booked but customers have not paid yet). This is why "cash is king" — businesses die from cash flow problems, not from lack of profitability.
Three types: operating cash flow (from core business activities) — the most important; investing cash flow (buying/selling assets) — usually negative for growing companies; and financing cash flow (borrowing, issuing shares, paying dividends). Free cash flow (FCF) = operating cash flow minus capital expenditures — the money available for shareholders.
Amazon was unprofitable for years but always had strong operating cash flow — because customers paid immediately while Amazon paid suppliers later. This negative working capital model generated billions in cash. Investors now focus on FCF more than earnings — companies with consistently strong FCF command premium valuations.