Adhoc testing : without test data r any documents 
performing testing.

Error Guessing: This is a Test data selection technique. 
The selection criterion is to pick values that seem likely 
to cause errors.

Adhoc Testing : Testing the application by tester who is 
having minimum knowledge of application information.so that 
his approach would be different to usual,there is chances 
of discovering uncovered bugs

Error Guessing : Its a Black Box Testing Technic,here the 
tester with his previous experience in 
application,domain,environment...etc, he is going to write 
test cases and execute.
simply it is   "what happens ,if ...

Adhoc means ones own way without following any guidlines.it 
is a type of testing without knowing the internal 
funcnality of the application.
it is a type of testing without using any testcase document.