A sales letter in video form. Most common format for high-ticket offers.
A VSL is typically a 5-30 minute video that pitches a single offer. Often it's just a voiceover with slides or text on screen, not a 'fancy' video. The format works because video holds attention longer than text does for most audiences, and you control the pace of information delivery.
For anything mid-to-high-ticket ($500+), VSLs consistently outperform text sales letters on cold traffic. They're also easier to record than write, for most people.