I was confused for some time about the JUNOS version numbers even after the new Quarterly release format (10.x releases and higher) came out so I thought I would share what I have learned.
The basics that you probably know
What does 10.1r1 mean:
10 = 2010 IE the year of release
.1 = The Quarter 1 feature release, each quarter adds new features
r1 = Tthe release/patch/hotfix level of the 10.1 release
What about 10.1s6 ?
S releases are service level release, most often a higher number than the last r the main difference is that they are not supported by NSM and also are not generally supported for very long, they get released faster than normal to fix critical bugs.. It is almost always best to migrate to a stable r once one is available.
You can't compare 10.0 and 10.1 they are different releases and each will get their own r updates for up to a year. 10.1r1 may contain bugs that where fixed in 10.0r3 and wont be fixed in 10.1 until r2 or 3
The higher the r in theory the more stable the release... I have heard from some sources that anything less than an r3 really isn't considered reliable. So you may be better off with a 10.0r3 than a 10.1r1 for example, unless you need a feature only found in 10.1 releases.
There are extended end of life releases... Special versions will be marked as EEOL this means instead of a years support for new r releases it will have more like 3 years.... 10.4 will be the next EEOL release and I believe moving foward the 4th quarter release will always be the EEOL release.
Feel free to correct me on anything but fairly sure all of the above is accurate.