Exploratory testing means testing the process without any 
specific plans and schedules.

u can say that its a one type of informal, creative testing 
not based on test plans or test cases. Testers may learn the 
software as they test it.

Exploratory Testing is an informal testing which is used to 
gain familiarity of the application.

When a tester test a application by exploring with his 
previous experience and based on the application tester 
writes test cases. This is exploratory testing

Suppose we didn't having any testing documents that r related 
to our application...and also we don't know the functionality 
of the application ...then this time we go for exploratory 
testing...in this type of testing we know the functionality 
of the application..

Exploratory testing  is an interactive process of 
concurrent product exploration, test design and test 
execution.

The tester designs and executes the test cases while 
learning the product.

Exploratory testing is useful when:

1. you are asked to test something on short notice.
2. test early in development process.
3. in situation with minimal document.

When a tester test a application by exploring with his 
previous experience and based on the application tester 
writes test cases. This is exploratory testing

It is the formal testing where we dont use test plan and 
test cases.when we get an application we have learn that 
application and we have to test by using our creative 
knowledge.