Having said that, you'll need to check your mortgage agreement for its assumption, movement, and due on sale clauses. Nevertheless, I suspect you asked this question as a prelude to another one on seller financing. Assuming I'm correct, I'll answer a question that you might have, but didn't ask.
Basically, you'd like to sell your property with seller financing, but you're not sure how the process works. The assumption works the way John described. The key to assumption is to ensure that your name gets taken off of the mortgage and title. You also need to determine whether you'll have the buyer to cash out your equity or finance it with you.
If it is, your asignee has to be qualified by your lender..
if it is important to you, make sure your liability is removed . In other words , you will want to be taken off the mortgage.
The lender has to give you the forms for assignment.