"Bulls make money, bears make money, pigs get slaughtered" is an old Wall Street saying that warns investors against excessive greed. The use of the terms "bull" and "bear" to describe market outlook is derived from the manner in which these animals attack. A bull tends to charge with its horns thrusting upward into the air, whereas a bear tends to swipe its paws downward to attack. These two distinct styles are metaphors for market activity on a whole. If the market has a positive outlook, it's considered a bull market. By contrast, if the market has a negative outlook, it's considered a bear market.

Bulls
A bull market is one marked with optimism and high investor confidence. During bull market periods, the economy is generally doing well, unemployment is on the decline, and stock prices are going up. Furthermore, the attitude associated with a bull market is that stock prices will continue to rise. In some regards, investing during a bull market is easy because the expectation is that stock prices will simply climb and remain high across the board. The problem, however, is that bull markets don't last forever, and they can often cause investors to lose money by holding overvalued stocks.

Bears
A bear market is one plagued with pessimism and low investor confidence. Bear market periods tend to coincide with looming economic recessions and are marked with falling stock prices. Investing during a bear market can be challenging because it's hard for investors to pinpoint which stocks will be profitable when prices are trending downward on a whole. In fact, some investors prefer to wait out a bear market and buy stocks only when they feel it's reaching its end.

Pigs
Pigs are investors whose goal is to make the most amount of money in the shortest amount of time. Piggish investors are known to either take on high degrees of risk or overlook risk in order to make a profit. They often make rash investment decisions and buy stocks without doing their proper due diligence. As a result, they tend to lose money, hence the adage that they inevitably get slaughtered. While bullish and bearish investors may have opposite investing styles, they each have the potential to make money if they manage to time the market correctly. Pigs, by contrast, are the most likely to lose money no matter the shape of the market because of their greedy attitude.

If you're bullish on the stock market and ready to invest, check out our Broker Center.

This article is part of The Motley Fool's Knowledge Center, which was created based on the collected wisdom of a fantastic community of investors. We'd love to hear your questions, thoughts, and opinions on the Knowledge Center in general or this page in particular. Your input will help us help the world invest, better! Email us at knowledgecenter@fool.com. Thanks -- and Fool on!