Versions of Asterisk
There are a number of supported versions of Asterisk available, including the current Long Term Support release 11, the previous Long Term Support release 1.8 and the Standard Support release 10. Additional branches include the Certified Asterisk branch of 11 and 1.8 and the "Digium Phones" branch of 10. These branches include additional support, not available in the mainline releases, for Digium's IP Telephones.
Mainline releases of Asterisk are made roughly every 6-8 weeks. Releases of Certified Asterisk are made roughly 3 times annually. Users new to Asterisk are encouraged to download the latest Long Term Support release, with the guidance that Certified Asterisk branches based on those releases have the least churn.
More information about the various versions of Asterisk is available on the Asterisk Versions wiki page. A complete listing of download options can be found on the Downloads Server. Information about installing Asterisk from source is available on the Installing Asterisk from Source Wiki pages.
Long Term Support (LTS) Releases
Long Term Support (LTS) releases are made from Asterisk branches where the focus has been on stability and user experience. New point releases include the latest bug fixes.
Standard releases are made from branches of Asterisk that received major new features. These branches are supported for a shorter period of time relative to LTS branches. Bug fix releases are made for one year, while security releases are extended for an additional year.
Test Releases are previews of the next versions of Asterisk. Each is in an alpha, beta, or a release candidate phase. Users who need to run the latest release for a bugfix are encouraged to test the test releases in their environments and provide feedback to the issue tracker.