"a formal assurance that certain conditions will be fulfilled, especially that a product will be of a specified quality"
"A written guarantee promising to repair or replace an object if necessary within a specified period."
A warranty is a type of guarantee; in the case of a product guarantee/product warranty, it's basically the same thing
- the company undertakes to repair or replace your goods if they go wrong.
Guarantee can also be used to express
"I promise, this plane will fly."
"I guarantee you, this plane will fly."