Defining Market Structures

Perfect competition
It's a market structure where large number of firms compete against each other to maximize their profit .There is free entry and exit to the market .a
All goods sold are identical and no consumer preference.  No particular firm has power in the market.
Example: stock market

Monopolistic competition
It's also a market similar to the perfect competition only the difference is in monopolistic market firms sell differentiated product and Consumers may prefer one over the other .So some firms have certain amount of market power.

Oligopoly
It's a market structure with a small number of firms in the market that result in limited competition. Oligopolies together can set prices and create barriers to entry and exit. There are only few firms that dominate the market with similar or differentiated products. Example :Gaming console where the market is dominated by Microsoft ,Sony and Nintendo.

Monopoly
Market where a single firm controls the entire market with the highest level of market power. As consumers do not have any alternatives, this firm can set its price much higher .The market has high barriers to entry and exit.