It's time to learn all about stocks, from what they are to all the different types available to what they can mean for your portfolio. Buy-and-hold investors invest in companies that have stood the test of time. Traders take a more active approach to investing, placing more emphasis on stock price movement than on the real value of the company. Regardless of which strategy you apply to your holdings, the same underlying rule applies: Know what and why you're buying (or selling) before you make any trade.