Debian is a collaboratively developed free operating system. Debian GNU/Linux is based on the basic system tools of the GNU Project as well as the Linux kernel. Debian offers a wide range of application programs and tools. Debian was founded in August 1993. Today, the project has over 1000 official developers. It is one of the oldest, most influential, and most widely used GNU/Linux distributions. Many other Linux distributions are based on Debian. The best-known Debian GNU/Linux derivative today is Ubuntu, from which the Linux Mint, which is also widely used, is derived.
Ubuntu has a server version. Debian lacks the server version. Debian uses SystemV style init, and inittab whereas
Ubuntu uses Upstart.
CentOS 7+, and Ubuntu 15.04+ both come with systemd. The special thing about systemd is that it has some tools for figuring out the buggy boot of dedicated servers. Systemd is the new init framework. It started with Fedora. However, most of us use SysV init style scripts. If your dedicated server reboots nicely with Debian then you do not think about this difference.
Please remember that restarting a dedicated server is a serious matter and certain users may want to avoid it. On Debian, you will not get so frequent a prompt to restart. Most Linux distributions require a reboot (almost once a month) to stay up to date with kernel and user-space security updates. Only Oracle Linux (Ksplice) allows avoiding a reboot. Oracle Linux (Ksplice) is like a fork of REHL or CentOS.
Debian has a root account enabled by default, whether Ubuntu will use a sudo account as primary depends on the web host. Debian has a release cycle which is not predefined (it is nearly about two years on average) but Ubuntu has a release cycle of two years for LTS. Debian has a special rolling unstable release named sid. Debian supports more architectures than Ubuntu.
Essentially Ubuntu is a copy-paste of Debian with some modifications. Most of Ubuntu’s packages are recompiled from Debian, so the configuration files such as Apache, MySQL will remain identical.