A bear is an investor who is pessimistic about the prospects for a market, a sector or a particular security. The expectation of a fall in a security or an index is described as "bearish". The opposite of a bear is a bull.
A bearish view (if it eventually proves to be correct) is as much a source of profitable trading opportunities as a bullish view. An investor can exploit expected declines by using derivatives (for example by buying put options) or short selling.
Many bearish strategies can be risky. Short selling and writing call options are particular dangerous as they can lead to losses linked to the extent of a price rise. There is therefore no ceiling on the possible loss.