Joomla 1.0, Joomla 1.5, Joomla 1.6, Joomla 1.7, Joomla 2.5, Joomla 3.0, Joomla 3.1 and Joomla 3.2. These are the major current and past Joomla versions. Some were supported by the Joomla project for years, others for just a couple monhts.
What version do you need to choose for a new website? What version is currently supported? For how long can I use the version I currently use? What about migrations and upgrades? STS and LTS support?
In this presentation Sander explains everything around Joomla versions and upgrades. Many things changed over the years and seem to be complicated. But it is actually as simple as Joomla 1, Joomla 2 and Joomla 3.