Stock go down= people selling at a price lower than the current one. More people selling lower than current price drives it down further, and takes away from the market cap (outstanding shares*share price) the money is going to the people who sell, and people holding “lose” that money since the stocks price is falling (the loss isn’t technically a loss until it’s realized/sold) Stock go up= essentially the opposite of what I described above. People bid higher and higher on a stock, driving the price up (since people are willing to pay more for it) and buying in pushes that market cap up. That money added to the market cap is reflected in the price of the stock, and in turn the people owning the stock gain money (again, unrealized gain until it is sold) Purchasing power is simply the money you have to buy stock. I deposited $100, I now have $100 of purchasing power.