The 'Revised^n Report on the Algorithmic Language Scheme,' often abbreviated 'Revised^n Report on Scheme' or simply 'RnRS,' documents are reports describing the Scheme language; they are the de facto Scheme standards. (The 'real' standard is the IEEE-standard, but it is out of date and not widely regarded.) The n stands for the number of revisions. The latest RnRS is r6rs. It is available in a number of formats, including PDF and HTML.
R5RS was the previous standard.