The term Adhoc Testing, also known as random testing, generally refers to a TYPE of testing that occurs without proper planning or documentation. Adhoc testing has 

• No documentation
• No Test cases
• No Test Design

Ad hoc testing is usually PERFORMED randomly without documentation or testing design and it is usually unplanned. Ad hoc Testing does not adhere to any particular structure and is done randomly on any part of the application to identify defects/bugs. When TIME is limited and exhaustive testing cannot be performed, adhoc testing may be conducted. The tester needs to have a thorough understanding of the system under test in ORDER to conduct effective adhoc testing. 

Example: Adhoc testing is cost-effective and can save you a lot of time; one example would be when the client needs the product by 4 PM today, but the development will be finished by 2 PM. With only 2 hours to work with, the DEVELOPER and tester team can test the system as a whole by taking some random inputs and checking for bugs.