Difference between 'guarantee' and 'warranty'

according to dictionary.com --

A guarantee is a promise or assurance, especially one in writing, that something is of
specified quality, content, benefit, etc., or that it will perform satisfactorily for a given
length of time: a money-back guarantee.
https://www.google.com/search?q=guarantee

A warranty is an act or an instance of warranting; assurance; authorization; warrant.

In law, a stipulation, explicit or implied, in assurance of some particular in connection with
a contract, as of sale: an express warranty of the quality of goods.

Also called covenant of warranty. a covenant in a deed to land by which the party conveying
assures the grantee that he or she will enjoy the premises free from interference by any
person claiming under a superior title. Compare quitclaim deed, warranty deed.

(In the law of insurance) a statement or promise, made by the party insured, and included
as an essential part of the contract, falsity or nonfulfillment of which renders the policy void.

A judicial document, as a warrant or writ.

A written guarantee given to the purchaser of a new appliance, automobile, or other item
by the manufacturer or dealer, usually specifying that the manufacturer will make any repairs
or replace defective parts free of charge for a stated period of time.
http://dictionary.reference.com/browse/warranty

they are indeed synonyms of each other, so can be used interchangeably in most cases...

You , the consumer, have a role to play in making markets function better. By specifying what you want as an informed purchaser you force the businesses to provide high standards of goods and services. So, understanding terms like warranty and guarantee become important.

There are differences between the two.

A guarantee is usually issued for subjective reasons like "satisfaction," whereas a warranty is issued for a product to cover defects or performance failures over a specified period of time. Some warranties are issued "pro rata" whereby credit is given for the amount of time in which a product was warranted to function but failed to do so. Automobile batteries are generally warranted in this manner.