A Testimonial is an official pitchman for a product. Franz Beckenbauer for example was a testimonial for the cell phone provider O2. Michael Ballack is the testimonial for ab-in-den-urlaub.de. More often than not, testimonials are relatively prominent (and because of that, relatively expensive too). But lesser-known athletes can be used as an effective testimonial as well - if they fit the advertised product.
For a testimonial to be succesful, it is essential for him/her to be seen as trustworthy.
Many testimonials are also spreading the message of their sponsor via their own social media channels (e.g. Facebook, Twitter, Instagram).