Since Groovy 2.0.0
Since Groovy 2.0.0 we follow the semantic version scheme. This means the next minor version after 2.0.0 is 2.1.0, the first bugfix version after 2.0 is 2.0.1 and the next major version will be 3.0.0.
Before Groovy 2.0.0
Before Groovy 2.0.0 we followed a version scheme where we had X.Y.Z, where X.Y was the major version, and Z the minor version. Bugfix versions where not really done, you had to upgrade to the next minor version for that. Since Groovy 1.0 we incremented only the Y for a new major version. The increment of X we wanted to leave for a very big breaking change, like a new Meta-Object Protocol. The last major version in these scheme is 1.8(.0), 1.8.1 is the first minor and bugfix version. The major versions in the past using this scheme are: 1.8, 1.7, 1.6, 1.5, 1.0. Each of them having around 10 minor/bugfix versions.
Official Major Version
The official major version is the current major version that should/can be used by the developers if they are not bound to a specific major version.
Maintenance Release Branch
Here we indicate a former major version's bugfix release.
How long is a major version maintained?
That depends on the users. Let's say we have X in maintenance and Y is the official major version, then if a new major version Z is released, Y goes into maintenance. Usually we make one or two more bugfix releases for X and then we discontinue it, unless there are strong requests to have certain things fixed for users that can absolutely not upgrade.