Newcomers to Linux world generally confuse or no idea about distributions. We call generally the operating system as Linux but actually Linux is just a kernel. So why we call all of them Linux because kernel is very important and without Linux kernel the distributions can not be exists. But reverse Linux can exist without distributions.
The Linux project is started as a kernel or low-level operating system. The student named Linus Torvalds started as a hobby and put his code into the internet for others to look, use or provide. This call fired a project where hackers all over the world to start contributing to the Linux project.
Linux Kernel provides operating system-level features and user-level tools, applications are provided by different communities, projects or corporates. Linux kernel provides following operating system features.
- Memory management
- Hardware Access
- File system management
- Network Device management
- Disk Drive Management
- Process Management
- User rights and privileges
- Peripheral like USB, Firewire management
And Linux kernel does not provide following tools, applications, and features.
- Browser like Chrome, Firefox
- File manager
- GUI Desktop
- Daemons or services
- Graphic design
Distributions provides user level tools, desktop environments, packages, graphic arts etc.
There are a lot of distributions in the wild. There is no exact count but over 300. Very little of them are very popular like Ubuntu, Debian, Fedora, Suse, CentOS, etc.
Distributions can base other distributions to make things better for them. For example, Ubuntu uses Debian as upstream distribution and get most of the packages from Debian. This gives Ubuntu less workload but some dependency on Debian. Also, Mint distribution uses Ubuntu as base. As you see the relationship between distributions is not so simple.
Distributions have some motivation like being fast, being eye candy, being the latest version, being easy to use, etc. This motivation makes them different. Almost all of them uses the same software.
More about different distributions can get from DistroWatch