No one really knows the exact origin of the terms "bull" and "bear" to describe the stock market, but their meaning is clear. The most important thing to know about these terms is that they describe long-term trends, not short-term changes. Bull and bear markets are usually measured in years.

A bull market is a rising market. In a bull market, investors are positive. The economy tends to be strong.

A bear market is a declining market. It tends to begin with a sharp drop in stock prices across the board. There is usually an eye in the storm, during which stock prices increase. But the storm returns, of course, and the bear market falls and falls and falls.
In a bear market, the economy tends to be weak. Unemployment increases. Consumers spend less, which results in lower business profits. As we've seen, this devalues a given company's stock. Investors tend to sell their stocks before the value decreases too much. Investors don't want to take risks because they don't feel good about their chances.