In Java, there are two categories of data types: primitive and object data types.

Primitive data types are the basic or fundamental data types in Java. They are also referred to as value types because they represent simple values. These data types include byte, short, int, long, float, double, char and boolean.

Object data types, on the other hand, are more complex and represent objects or instances of classes. They are also called reference types because they hold a reference to the object rather than the object itself. These data types include Strings, Arrays, Classes and Interfaces.

One major difference between primitive and object data types is how they are stored and manipulated in memory. Primitive data types are stored on the stack, while object data types are stored on the heap.

Another key difference is the level of default initialization each data type receives. For primitive data types, Java initializes them with a default value, such as 0 for integers, false for booleans, or null for object references. Meanwhile, object data types do not have a default value, so they have to be explicitly initialized either through constructor invocation or by assigning a value to them.

Additionally, primitive data types are passed by value, meaning that when passed to a method, a copy of the value is created and passed, leaving the original variable unaffected. However, object data types are passed by reference, meaning that any changes made to the object within the method will be reflected in the original object outside the method.

Overall, understanding the differences between primitive and object data types is important when working with Java as it affects how you declare and use variables, as well as their behavior.