Bull and Bear Market are terms used for describing the phase of stock market.

• Bull market: Rising index / stock prices, investor optimism, strong economy. Generally, wealth is made by investors during a bull market.
  
  
• Bear market: Falling index / stock prices (20%+ decline), pessimism, economic slowdown. Generally, gives an opportunity to buy quality shares whose prices have fallen.

Markets move in cycles, and long-term investors should account for both. Investing in mutual funds through a Systematic Investment Plan (SIP) generally aids in long-term wealth creation, as it allows you to accumulate more units during bear markets and benefits from rupee cost averaging.