Warranty:
a written assurance that some product or service will be provided or will meet certain specifications

Guaranty:
a collateral agreement to answer for the debt of another in case that person defaults

Guarantee:
n] a written assurance that some product or service will be provided or will meet certain specifications
a pledge that something will happen or that something is true; "there is no guarantee that they are not lying"
a collateral agreement to answer for the debt of another in case that person defaults
[v] promise or guarantee
[v] give surety or assume responsibility; "I vouch for the quality of my products"
[v] make certain of; "This nest egg will ensure a nice retirement for us"; "Preparation will guarantee success!"