I'm not sure what you mean or which versions you're referring to.
Like every project there is the 'master' branch and nightlies which is latest and greatest (and probably the least stable, but still go over heavy CI tests).
The oVirt project reached a point when the next stable major version (4.0) is nearing GA, and part of the process is to branch it to create rpms for it, in order to continue developing 4.1 (now master) and not to merge new features that may risk 4.0 stability the new branch has to be created.
You as user/developer has the choice to choose which version to use, according to you needs:
for e.g, at this time point, you choices are:
1. Use current stable: 3.6 rpms - most stable, only gets specific fixes and not major features - recommended for a production usage and if you don't need any of the new features [1]
2. Use current RC candidate for next major 4.0, which should be the next stable version soon and replace 3.6 as the latest stable version [2]
3. Use nightlies for every version - either 4.0, master, 3.6 - use at your own risk since these are not official releases and not going via extensive QA, although the automated tests for each version
are improving over time and make the nightlies more and more stable. [3]
Just to help better undertand, if you'll make the same decision in one more months for e.g, after 4.0 is GA, you choices will be:
1. use current stable 4.0 (unless you really want to use older 3.6)
2. use nightlies
So the extra versions are temp version that are part of the development process and actually gives you an oppertunity to check out new features that are not available yet in a formal stable version.
I hope it gives some info on the diff between all the versions.