From their respective web sites:
Debian is a free operating system (OS) for your computer.
openSUSE is a free and Linux-based operating system for your PC, Laptop or Server.
Fedora is a Linux-based operating system that showcases the latest in free and open source software.
Gentoo is a free operating system based on either Linux or FreeBSD that can be automatically optimized and customized for just about any application or need.
Super-fast and great-looking, Ubuntu is a secure, intuitive operating system that powers desktops, servers, netbooks and laptops.