To add to what has been said, the R term is one that was used by LGB for their track radius measurements. R's 1,2 and 3 correspond pretty much to Aristocraft and USA track radius measurements but LGB actually measures in metric whereas Aristo/USA is measured in feet. Since this pretty much only applies to sectional track we only have to look at six or seven different curvatures. R1, R2 and R3 are the most common. It is also prudent to know that the wide radius turnouts from LGB have a different curvature than the ones from Aristo/USA.
Note that Piko also uses "R1, etc., " but they are not the same as LGB's. As a rule, just forget about any cutesie nomenclature, and go by the actual measurements. Each manufacturer states what their curves are on their web site. Be careful to note whether the number is the radius or diameter, though. That varies depending on manufacturer.
Yep, Dan, you're absolutely correct about the metric part but how do you get "larger" measurements from a 2ft. radius curve? This is why we interchange R1 with 2ft. and R2 with 2.5 and so on. Note that I use the term "correspond" rather than saying they are the same. In my opinion, when talking about such a tight radius, any difference is so negligible as to be irrelevant. As we go larger though, the differences are magnified and then it does matter!
10' diameter (5' radius) is a good, solid minimum that will easily accommodate 95% of what's available today. You could go down to 8' diameter (4' radius) and run probably 70% of what's available, but the large stuff will look funny doing so. You can go smaller without too much difficulty, but you begin to limit yourself to what will run reliably over the track. Much of the new stuff--especially in the 1:20 realm--requires an 8' minimum diameter (4' radius).