Investing in stocks can be very costly if you trade constantly, especially with a minimum amount of money available to invest. Every time that you trade stock, either buying or selling, you will incur a trading fee. Trading fees range from the low end of $10 per trade, but can be as high as $30 for some discount brokers. Remember, a trade is an order to purchase shares in one company - if you want to purchase five different stocks at the same time, this is seen as five separate trades and you will be charged for each one.
Passive investing is what long-term investors do — and the intent behind their actions is very different. Their approach to buying and selling stocks is considered passive in that they tend not to transact often. Instead of relying mostly on technical analysis (as active traders do) and trying to time the market, passive investors use fundamental analysis to carefully examine the strength of the businesses behind the ticker symbols and then buy shares with the hope that they’ll be rewarded over years — decades, even — through share price appreciation and dividends.
People trading stock will prefer to trade on the most popular exchange since this gives the largest number of potential counter parties (buyers for a seller, sellers for a buyer) and probably the best price. However, there have always been alternatives such as brokers trying to bring parties together to trade outside the exchange. Some third markets that were popular are Instinet, and later Island and Archipelago (the latter two have since been acquired by Nasdaq and NYSE, respectively). One advantage is that this avoids the commissions of the exchange. However, it also has problems such as adverse selection. Financial regulators are probing dark pools.