Prototype is the screens of the build/project.Itis not
operatable because prototype is given to the testers where
the build is under construction.

Prototype is nothingbut screen shot of the application,this
prototypes are non functional screens,validate the
prototypes to meet the customer expectations is called the
Prototype testing.

ITS NOT MUCH CLEAR CAN ANYONE EXPLAIN ME ...

In this type of testing, a module/functionality is added to 
the application in each step...this is calld proto-type, 
they test it and if it works well(proto-type testing), 
another module/funcionality is added...

There are 2 type of proto-typing:
Throw-Away Proto-type: After meeting the requirements,
the completed application is thrown-away and started from 
design phase freshly/correctly.
Evolutionary proto-type:After meeting the requirements,
Product goes into deployment.

As the name says Prototype, its a prototype of the original 
application but created earlier than the original 
application.

Prototype testing is to test the model/dummy application 
crated by developers or sometime given by customers, 
example: in car manufacturing company creating the clay 
model/3D models (Prototype) and designs of car than live 
model (Real car)

OR
Prototype testing is a type of testing to test the pre 
created/dummy model application or dummy application of 
real application that can be referred to check the 
consitency and flow of the field values/data while testing 
the real application. 

This prototype model is created by developers on request 
from the inhouse marketing team/managers to show it to 
customers/clients to satisfy the functionalities/design 
part and look and feel of the application and to get it 
approved and sign off.

Hope this helps you or if you think its wrong pls update me 
so that i can get more knowledge from you, I appreciate 
your time.

Best Regards,
Umesh Gowda