Details about check-in and check out can be read in following section
http://siebelunleashed.com/category/siebel-tools/
In this post I will just be answering the question
What is the difference between UNDO CHECK OUT and GET?
As this is a subjective answer so I will write my usual disclaimer 
The views expressed in this post are mine and only mine. Use them at your risk in an interview.
UNDO CHECK OUT: Is the process of reverting back the changes that you have done and also unlocking the object on server. So, two things happen when you do an UNDO CHECK OUT.
- Lock is released on the server for that object
One typical example where you would require doing an UNDO CHECK OUT is you have worked on some requirement and that requirement has been cancelled. In that case you would NOT want to put your changes to server so you would do an UNDO CHECK OUT. Follow the steps given below to perform and UNDO CHECK OUT
- Click on Tools –> Check In
- Select the object that you want to UNDO
- Click on UNDO CHECK OUT
GET: It is process of getting the copy of an object on your local system. It doesn’t not involve locks which means that neither the object is locked and nor the object lock is released. So, when you do a get the object on your local is updated with the latest copy from the server.
Have more questions in your mind???
Post it in comments and I will answer it for you.
As pointed out by Varun in comments UNDO CHECKOUT does NOT result in local tools object to revert back in orignal mistake. Sorry for that mistake.
