Exploratory Tests are categorized under Black box Tests that are aimed at testing in conditions when sufficient time is not available for testing or proper documentation is not available.

Exploratory testing is 'Testing while Exploring'.When you have no idea how the application works,exploring the application with the internet of finding errros can be termed as Exploratory testing.

Performing Exploratory Testing
This is one big question for many pepole.The following can be used to perform the exploratory testing:
1.Learn the Application.
2.Learn the Business for which the application is addressed.
3.Learn the technology to the maximum extent on which the application has been designed.
4.Learn how to test.
5.Plan and design tests as per the learning.