tufte-css-jekyll is a theme based on tufte-css, which in turn implements the design ideas of Edward Tufte in CSS, and tufte-jekyll (but tries to stick more closely to the original tufte-css styles than the latter).
Note that this is a full-width layout. This was accomplished by including
layout: full-width in the YAML front matter for this page, i.e., no sidenotes or marginnotes!
If you want to know what Tufte style comprises, have a look at the Tufte CSS page.