I can choose from two possibilities of new Visual Studio installation
But I'm a little bit confused what do these versions mean.
Visual Studio 15 is the version after Visual Studio 2015. The "15" is effectively the mostly-internal version number, and the "2015" is the branding version. Yes, it's very confusing.
Other internal/branding mappings:
Unless you want to experiment with prerelease software (which can be fun, don't get me wrong) you want to install Visual Studio 2015... and update 2 is the most recent release of that.
We don't yet know what the branded name of Visual Studio 15 will be - mostly because we don't know when it's going to be released.
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