A "bull" (or a person who is "bullish") expects the prices of stocks or other trading instruments in a given market to rise. A "bull market" is a financial market in which prices are rising or are expected to rise.
The greatest bull market in U.S. history began in the summer of 1982. At that time, relatively few people owned stocks, and the stocks were cheap because they were considered highly risky. The Standard & Poor's 500 peaked in March 2000 in an stock bubble; the masses of investors who had gotten into the market during the bubble then saw their stocks go down dramatically in 2001.