A long-form written pitch, typically for a single product, with no distractions.
A sales letter is a webpage (or printed letter) designed to take a cold prospect from skeptical to sold on one page. No nav, no sidebar, no other links, just a headline, the body, and a single call-to-action repeated throughout. Usually 2,000 to 8,000 words. They're 'long' because they need to be; a high-price product can't be sold in a paragraph.
If you're selling anything at price where 'think about it' is a real risk, the sales letter is the canonical format. Master it and you can move any product you actually believe in.