There are three versions of Joomla available at the moment.
With three versions around, we decided to make one simple, straightforward video that would explain how Joomla versions work and which one you should use. Here's that video:
So the key piece of advice in the video is this:
We are going to continue teaching Joomla 2.5 in our live classes until next year. We're also going to keep on producing online classes for Joomla 2.5.
In the next few months, we will add classes on Joomla 3 of course. However, we'll really increase our output next year, after Joomla 3.5 is released and becomes the recommended version.
Here's a larger image of the Joomla release cycle that was shown in the video: