Linux is ONE OPERATING SYSTEM (OS), but unlike Windows or Mac, it is available as a multiplicity of systems branded as separate OS-es, different in the sense that they are available for download as different BUILDS, that is systems containing the Linux kernel packed by different companies or communities with a variety of DESKTOP ENVIRONMENTS (DE, or simply 'desktops'), applications, drivers, update sources (also called 'repositories'), etc. A desktop environment is a graphical user interface (GUI) to use in interacting with your computer's operating system (OS) instead of using the command-line interface.
The kernel itself may be different in the sense that different builds may use different VERSIONS of the Linux kernel. Also, Linux distributions differ in relation to the kernel version in the sense that some use one or other version of the kernel for their stable releases, while others (called rolling releases) update continually the kernel itself. But for the end user, especially for the beginner and intermediate user, the differences between different Linux OS-es will be in fact differences between the desktops environments used by the specific builds.
For example, I will mention some Linux operating systems and then define some differences between them.
The most known Linux OS are Ubuntu and Linux Mint. Other Linuxes are Manjaro, Elementary OS, Fedora, and many others.
But while I mentioned 5 Linux operating systems, in fact, three of them, Ubuntu, Elementary and Linux Mint are all based on the Ubuntu build, which is maintained by the company Canonical. Ubuntu is itself based on the Debian Linux DISTRIBUTION. Debian is available as a separate build itself, a rolling release, one using only open source software, meaning that it lacks a lot of drivers, for example. Ubuntu, which is not a rolling release (providing short-time and long-time versions of its distribution) provides the a Debian-based build with some various drivers and applications, and also its own repository base (update sources). Taking advantage of all these, Linux Mint provides its own build, which means a streamlined Ubuntu (therefore non-rolling) with access to the Ubuntu repositories to install and update applications. (For this reason, many users consider Linux Mint as one as the best Linuxes, as it is a Linux improved both by Debian, Ubuntu, and the Mint company and community. But some who dislike Ubuntu, will also dislike Linux Mint.)
Ubuntu has a large community is provided with different desktop environments. While Ubuntu is the basic OS in all the Ubuntu family, the distribution branded as Ubuntu proper uses its own desktop environment, called Unity: that is what you look and feel when in contact with Ubuntu. The difference is in the appearance, but also in the different things you can do and the ways you can do them. This means that the memory use is different.
KDE is the DE used by Kubuntu. KDE and Unity are the fancier and, as a consequence, the heavier on resources in the Ubuntu. Xfce is a DE made to be used on computers with lower resources while keeping a rather stylish appearance - and is used in Xubuntu. LXDE - used in Lubuntu - is the lightest of the family - for even older computers theoretically, but keeping a feel that in my opinion is still more stylish than that of Windows XP, not to mention the speed. But while Unity is related to Ubuntu and Cnonical, KDE, Xfce and LXDE are only loosely related to Ubuntu/Canonical.
Ubuntu-proper = Ubuntu OS + Unity DE
Kubuntu = Ubuntu OS + KDE
Xubuntu = Ubuntu OS + Xfce DE
Lubuntu = Ubuntu OS + LXDE
Linux Mint also uses KDE and Xfce as the desktop, but also Cinnamon and Mate. Lately, there is a Mate edition of Ubuntu too.
Manjaro, on the other hand, is not based on Ubuntu, and not even on Debian, but represents a separate rolling distribution of the Linux kernel, with separate repositories. It is based in fact on Arch Linux and its repositories. Manjaro uses mainly KDE and Xfce as desktops.
Elementary OS is based on Ubuntu build and repositories, and therefore not a rolling release (in fact is one of the slowest in providing new releases) but uses its own desktop environment (Pantheon) and file browser. A user that is familiar with Xubuntu will find the same method of installing applications in Elementary, but otherwise this OS will look more different to him that does Manjaro-Xfce.
Also, Xubuntu will feel more familiar (if not identical) to a Linux Mint Xfce user than to a Ubuntu/Unity user. Manjaro Xfce will also feel familiar, until he tries to install programs: then, the AUR (Arch) repositories have to be used instead of the Ubuntu ones.
While Linux is one kernel, some differences appear between distributions depending on versions, rolling release or not, but also some new similarities appear based mainly on the desktop environments.
HERE is a large Linux distribution timeline