A laptop vs a desktop. A cupcake vs a triple layer cake. Mini-Me vs Dr Evil. A research letter vs an original investigation. All are smaller but fully functional cousins of the larger and flashier model.
Research letters in JAMA report results of studies, just as original investigations do. They have introductions, methods, results, and discussion sections, just like original investigations. They are peer reviewed and subject to stringent editorial review as well. But a research letter contains a maximum of 600 words and 6 references, as opposed to 3500 words and 40 references in an original investigation, and at most 2 tables or figures, as opposed to 5 or more in a longer article. Research letters do not have abstracts and ordinarily do not have supplementary web-only materials, in contrast with other studies published in JAMA.
Zylke JW. Research Letters in JAMASmall but Mighty. JAMA. 2013;310(6):589-590. doi:10.1001/jama.2013.8102