What is the Difference between a Bull Market and a Bear Market?

When demand for stocks is generally rising (pushing prices higher as the number of would-be buyers climbs), we are in a bull market period. When demand for stocks falls and prices slump, we call it a bear market. Both bull and bear markets can last months, years, or even decades, but nobody has found a way to reliably predict when they will begin or end. (Sad, but true.)