Rate classes can be viewed as more general way to define the prices for your products as many entities can share the same rate class. For example you could create different rate plans for different types of customers, different rates for different uses, etc. These are the plans that you can assign to your accounts when settings them up, as well as what you can assign to Pricing Plans.
Default Prices - Last line of rates used by the system if no pricing plans or rate classes are in effect.
Pricing Plans -Pricing plans allow you to set more granular levels of pricing specifications for customers. These are based off rate classes or completely custom. Please follow the link for more information.
Contract Rates - Contracts allow you to refine the pricing of existing products in the system relative to what you want to take effect to the customer.