What is the main difference between Collection and Collections in Java?

Collection is a base interface for most collection classes, whereas Collections is a utility class. I recommend you read the documentation.

Are you asking about the Collections class versus the classes which implement the Collection interface?

If so, the Collections class is a utility class having static methods for doing operations on objects of classes which implement the Collection interface. For example, Collections has methods for finding the max element in a Collection.

The Collection interface defines methods common to structures which hold other objects. List and Set are subinterfaces of Collection, and ArrayList and HashSet are examples of concrete collections.