Each release fixes various bugs and vulnerabilities as well as bringing some minor enhancements in usability. Note that improvements and bug-fixing has ceased for 1.9.x as focus shifts to Moodle’s most advanced releases. General support for 1.9 will be available through June 2012 [http://docs.moodle.org/dev/Updates#Moodle_1.9].
More information for each release can be found at
To download any of the versions, including version 2.2 visit http://download.moodle.org/