distrowatch.com is a useful website for comparing distributions and also for linux related news articles.
It also shows the most popular distros for a period of time which defaults to 6 months but can be changed to as little as 7 days.
Popular distros based on Debian include Ubuntu, Mint and Kali. Debian uses the Advanced Package Tool (APT) to manage software packages.
Popular distros based on Red Hat include CentOS, Fedora and Mandriva. Red Hat uses the Red Hat Package manager (RPM) for software package management.