I've not used Navisworks but I believe that it allows you to import & combine 3D models to for instance check that parts fit together OK. You can also use it to view, mark up & add comments to CAD files in various formats. Navisworks can not be used to create CAD files.
Inventor is used to create 3D CAD models, assemblies & drawings.
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I didn't have a really important life, but at least it's been funny (Lemmy Kilminster 1945-2015)