One of the most common ad types, the Testimonial features testimony from an expert in a certain field (a doctor for a pill, a plumber for a toilet cleaner, etc). The expert will speak on the virtues of the product ("I recommend this item to all my customers") while really saying nothing about the quality of the product.
Will sometimes feature a side angle of the interviewee, so it looks like they're being interviewed by a third party.
A particularly dubious version is used to advertise Sensodyne tooth products in the UK, wherein someone credited as, say, "John GSK Dental Market Researcher" tells us that dentists see "acid erosion" as a growing problem and Sensodyne toothpastes are an effective countermeasure. GSK being the company behind the Sensodyne brand. That's right, their own marketing department provide the testimonials for their product's efficacy. To top it all off, the dental research behind acid erosion is heavily sponsored by GSK itself.