MySQL and PostgreSQL compatible.

I've used both in production, with all sorts of advanced features, and (to my surprise) I've never yet found a difference.

I would have a hard time leaving Aurora though. Not because of normal "vendor lock-in", but because of the operational worry-free scaling, backups, replicas, point-in-time restores etc. that it offers. Basically, I'm "stuck" on Aurora just because no one else offers MySQL/PostgreSQL databases like that.

AFAIK It uses the MySQL frontend with a heavily modified InnoDB engine that writes to their shared-disk storage.