What are the different types of price discrimination that can be employed?

First-degree price discrimination, alternatively known as perfect price discrimination, occurs when a firm charges a different price for every unit consumed. The firm is able to charge the maximum possible price for each unit which enables the firm to capture all available consumer surplus for itself. In practice, first-degree discrimination is rare.
Second-degree price discrimination means charging a different price for different quantities, such as quantity discounts for bulk purchases.
Third-degree price discrimination means charging a different price to different consumer groups. For example, rail and tube travelers can be subdivided into commuter and casual travelers. Third-degree discrimination is the commonest type.
Price discrimination can only occur if the following holds:
The firm must be able to identify different market segments, such as domestic users and industrial users.
Different segments must have different price elasticities of demand.
Markets must be kept separate.
The firms must have some degree of monopoly power.