In Java, null is a reserved keyword for literal values. It is a special value that represents the absence of a value or reference. It is neither an object nor a type, but a literal. It is used to indicate that a variable or object does not currently have a value assigned to it. null is the default value of reference-type variables. It can be assigned to any reference type, but not to any primitive type such as int or boolean. Typecasting null to any reference type is fine at both compile-time and runtime and it will not throw any error or exception. However, it is imperative to note that null can only be assigned to reference types.

null is used as a special value to signify an uninitialized state, termination condition, non-existing object, or an unknown value. It is also used to indicate that a method does not return any value, known as a "void" return type. In addition, null can be used as a default value for optional parameters in a method.

To check whether an object or a string is null or not, we use the comparison operator ==. In programming, we usually need to check whether an object or a string is null or not to perform some task on it.

It is important to note that null can cause the infamous NullPointerException when developers forget to initialize their variables before calling them.