GUL TESTING : GUI Testing is nothing but to check the look 
and feel of the application.

UI TESTING : UI Testing is nothing but the user interface 
testing which is done in front of the user.

Thank you rajesh.
UI is, done in front of the user----then Is there any 
difference between User acceptence (either Alpha or Beta)  
testing and UI (testing which is done in front of the 
user... as u said)?. please clarify me

siva

User Interface always not in the graphical in nature. i 
mean it need not be a web based application always.
In my point of view both GUI and UI tests are same as test 
process is concern.

There really is no difference between the two.  As the last 
writer stated, not all user interfaces are 'graphical'.  
You could be testing a web service or an API where in both 
cases, there may not be a 'GUI' to enter values into.  
You're still testing the required inputs and the expected 
outputs, but with GUI's, there's an additional aspect to 
your testing.

In a GUI, you may be testing requirements like color 
scheme, button naming conventions, drop down menus and 
content on top of testing the required inputs and expected 
outputs.  In a plain vanilla UI, you are testing the same 
things minus the GUI elements.

If an interviewer gives you a questions like this, they're 
asking if you would prefer to have ice in your soft drink 
or frozen water.

So, if you get this question in an interview.  Be careful.  
If they know what they're talking about, they may just be 
checking to see if you make something up or if you really 
know what you're talking about.