So I want to learn using Solaris [Diskgroups, etc.]. I recently found out that there is an OS named OpenSolaris:
Is it a good point to learn Solaris? Or it's different then Solaris? How much it differs from it?
You can read the history of OpenSolaris from that article you link, and a bit on the main Solaris page on Wikipedia. That tells you that indeed, OpenSolaris is/was very close to plain Solaris.
Given that some modules - the core in particular - are not published as open source code anymore by Oracle, OpenSolaris (or the other open source derivatives) is likely to drift a bit from Solaris but if it's just to get started, it's still fine.
Also note that you can download Solaris 11 from OTN, including a live CD version and pre-built VM images for x86. If you check the FAQ for these downloads, you'll see (near the end) that they can be used without a support contract for non-production uses – read that section and the licensing requirements carefully to make sure it applies to you, as you should do with any licensing agreement.