1. Other Market Structures

2. Monopolistic Competition
Monopolistic competition is probably the most common form of business competition in the world.
Barriers to entry are low
There is some Product Differentiation
There is some control over prices (because of differentiation) or, Nonprice Competition, where style, features, service, or quality is a factor
Many sellers and many buyers
There are many examples of these kinds of businesses, like retail stores, or gas stations. They tend to have some kind of brand image, and differentiate themselves by service, product quality, or features.

3. Oligopoly
An oligopoly is a situation where there are a few firms dominate the market
Size of the firms allow for Market Power, based on Market Share
High barriers to entry (usually high start up costs)
Few sellers, many buyers
Products can be commodities (standardized products) or differentiated
In these cases, people are attached to a brand, like Pepsi, but if the prices rise significantly, they may choose Dr. Pepper instead.

4. Price Collusion and Cartels
When there are few suppliers for a good, it is possible for them to get together and set prices for the benefit of all the suppliers. This is called price fixing, and collusion. These practices are illegal, since they undermine competition in the market.
Cartels are a form of open agreement between firms to set prices, such as OPEC (Organization of Petroleum Exporting Countries). While illegal in the U.S., OPEC countries agree on the limits of each country to pump oil, thus limiting supply and raising prices.