difference between Entity Backed records and Process Backed records?

This is about the source of data for your Appian Records. There are 3 possible sources:

• Entity-backed - the data for these comes directly from a database table or view - i.e. a database "entity"
• Process-backed - the data for these comes from Process instances in your Appian application environment 
• Service-backed - the data for these comes from the result of an Expression Rule - which in turn could come from any number of different sources e.g. web services, database tables, or a combination of these