[Veratyr]

DaVinci Resolve is mainly for colour grading and basic shuffling around of video and audio. It's also a very advanced (read difficult) piece of software and is very much not aimed at home movies.

I wouldn't recommend it as an iMovie alternative.

The free version is free as long as you have it. There's no registration or keys or anything. There is the chance that future versions won't be free though.

The main differences between the free and paid versions are mainly collaboration stuff like sharing things between systems and doing things remotely. There are a few other things like real time noise reduction and automatic 3D alignment but nothing a home user is likely to notice.