The terms of an endorsement deal can vary a lot and, generally neither the company nor the person endorsing the product will make the details public.
It can range from the artist being provided with instruments/amps/strings etc. for free, or being able to purchase the products for a reduced price or at cost or wholesale.
In return the artist will appear in adverts/promotional material and use the product in live shows, on TV and plug it in interviews (or some combination thereof).
Sometimes the artist has to agree to use that company's product exclusively, sometimes they can use other stuff too.
It all depends on what is negotiated between the two parties - there is no such thing as a "standard" endorsement deal.
Sarah Jory endorses WBS Steel guitars.
http://www.wbssteelguitars.com/testimonialsus.html