A sales letter is a piece of direct mail which is designed to persuade the reader to purchase a particular product or service in the absence of a salesman or saleswoman .
It has been defined as "A form of direct mail in which an advertiser sends a letter to a potential customer.
"[1] It is distinct from other direct mail techniques, such as the distribution of leaflets and catalogues, as the sales letter typically sells a single product or product line, and further tends to be mainly textual as opposed to graphics-based, although video sales letters have become increasingly popular.
Because of the direct response nature of sales letters, they can be carefully tested on an ongoing basis to determine which version performs best in terms of converting readers to customers.
This allows the marketeer or copywriter to confirm which headline, body text or graphic design converts best.