Let's use an analogy to describe Linux distributions.
Think of the OS as a vehicle.
First, we'll start with its engine—that would be the kernel. Just as the engine makes a vehicle run, the kernel is the most important component of the Linux OS.
Because the Linux kernel is open source, anyone can take the kernel and modify it to build a new distribution.
This is comparable to a vehicle manufacturer taking an engine and creating different types of vehicles: trucks, cars, vans, convertibles, busses, airplanes, and so on.
These different types of vehicles can be compared to different Linux distributions.
A bus is used to transport lots of people.
A truck is used to transport a large number of goods across vast distances.
An aircraft transports passengers or goods by air.