I think it is nothing but gui testing, where we test the
user interface, user friendly ness, and look and feel  of
the application. If i am wrong please correct me

HI 

Interface testing has own importance in web base 
application ,like web base application divide in layer or 
tier 
1 - web      layer
2 - app      layer
3 - database layer

here we chacking interface between in this layer

if all layer in cluster 
we are able to take benifit for Cluter or not 
some time application ia install in cluster environment , 
but they are not taking cluester benefit due to cluester  
service is not working , cluster licence is expaire , heart 
bit , is not working

Interface Testing is not focussed on what the components 
are doing but on how they communicate with each other, as 
specified in the "System Design". The "System Design" 
defines relationships between components, and this involves 
stating:

What a component can expect from another component in terms 
of services. 
How these services will be asked for. 
How they will be given. 
How to handle non-standard conditions, i.e. errors.

in my opinion it is not butthe operation that are operated
through straight way...in this we can test any system online