Mainly a VDOM with some convenience features.
Ranked in these QuestionsQuestion Ranking
Pro Pretty light-weight
Inferno weighs in at 9kb gzipped, which is light-weight.
Pro Fast performance
Inferno is one of the fastest UI libraries around and widely considered the fastest.
Use it however you want in a framework of your own custom design. When things change in the industry, swap things out instead of being locked in by someone else's design.
Pro React compatability
Using the Inferno compatibility package ("inferno-compat"), Inferno can support the vast majority of React codebases.
Con Some React components may not work with Inferno
Inferno and React have different public and private APIs. If 3rd party components use a private API then it's almost certainly going to break when you use it with Inferno.
Once React Fiber is implemented, even libraries that are currently working will break and will not be supported by Inferno.
Con Not very popular
Which can hinder one's opinion of its future, but the future of all "frameworks" is to break things into smaller pieces, so inferno very well might get used by the big names in the future.