What's the difference between active record and ActiveRecord ?

ActiveRecord Is ruby on rails module that is an interface to the database. Makes programmers life easy by giving functions like find instead of using sql where.

As for active record i think you mean the current database entity being worked on. There is no such specific terminology 'Active record' is associated with

ActiveRecord is a Rails module for managing database entities in a programmer-friendly way. What you refer to as "active record" could be a particular record, say, in a database, that is currently "active", perhaps because a user has selected that record or some such way of selection.

In Ruby on Rails, ActiveRecord is an ORM framework. We use it to map real world object to database table without using the SQL directly. 

There are no difference

you might meant ActiveRecord and ActiveModel

ActiveRecord: Main function is associates a class to the database

while ActiveModel: includes things like validations

Best wishes