NVivo is software for Qualitative Research from QSR International. Many people consider it their favorite because it is so powerful and provides organization for the entire project. The learning curve is small to do basic tasks, but there are many advanced features that can take longer to get accustomed to.
NVivo 1.0 (released March 18, 2020) was released for Windows and Mac. As previously, the Mac version has fewer features. QSR also released Collaboration Cloud for sharing projects (within OS only). The Plus and Pro versions from v12 have been combined.
NVivo 12 (released March 20, 2018) for Windows has 2 versions. Pro has the basic features researchers need. Plus has additional automation and social networking capabilities. NVivo for Mac OS is also available, with somewhat fewer features than the Windows Pro version. The Starter Edition from v11 has been discontinued.
1. The documentation from QSR is great, and is the recommended way to learn NVivo by reading.
Start with the Using NVivo section, and follow links to specific documents in the other sections as relevant.
2. For more in-depth coverage, I would recommend the book Using Software in Qualitative Research by Ann Lewins; Christina Silver. It is not specific to any software, and thus will be useful regardless of the version you use, and remain so even as the softwares change (NVivo has been releasing a new version about every 2 years).
These are wonderful materials and should be the first stop.