To quote our friend, Matt Duclos of Duclos Lenses, Leica R's consist of the following families: "Summicron is f/2.0, Summarit is f/2.5, Elmarit is f/2.8, and pretty much anything slower than that is an Elmar. "
Matt also put it best when describing Leica R's, so I'll let him do it.
"The glass was top of the line and second to none when it was new. Even by todays standards, the Leica R lenses easily hold their own in a world of nano crystal coatings and optical stabilization. The images these lenses produce is, for lack of a better description, dreamy. The details are held extremely well while still allowing for that warm softness that has been associated with film and motion pictures for so long. A lot of people will say that they are reminiscent of the look that you get with Panavision. That’s probably because Panavision used very similar Leica glass for a lot of their own in-house lenses."