Question:What are the functions of num_rows() and num_fields()? 

num_rows() function will return the total number of rows returned from a query.
num_fields() function will return the number of fields returned by the query.

Question:What is the purpose of active record caching? 

It enables us to save a query for use later on in our script execution. Usually, an SQL query is reset after it has been completed. With Active Record cashing we can prevent this reset and reuse queries easily.