Once a company has gone through its IPO, investors buy and sell issued shares through intermediaries in the secondary market, which includes the traditional and electronic exchanges, as well as the over-the-counter (OTC) market.
Buying long is the traditional way to buy stock. When you hear your grandpa talk about the stocks he bought 50 years ago, he's talking about buying long. The stock you bought last year and held was also bought long.
While most individuals invest in the market when they believe a stock's price will go up, some sell short because they believe it will go down. To sell short, investors open a margin account, borrow shares from their broker and sell the shares on the market.