Define Checked and Unchecked exception.

A checked exception is one, which a block of code is likely to throw, and represented by throws clause.It represents invalid conditions in areas outside the immediate control of the program (invalid user input, database problems, network outages, absent files).
In Java it is expected that a method 'throws' an exception which is a checked exception.They are subclasses of Exception.

While unchecked exceptions represent defects in the program (often invalid arguments passed to a non-private method).

According to definition in The Java Programming Language, by Gosling, Arnold, and Holmes,"Unchecked runtime exceptions represent conditions that, generally speaking, reflect errors in your program's logic and cannot be reasonably recovered from at run time." They are subclasses of RuntimeException, and are usually implemented using IllegalArgumentException, NullPointerException, or IllegalStateException

It is somewhat confusing, but note as well that RuntimeException (unchecked) is itself a subclass of Exception (checked).