Say it like this

A computer whose whole job is holding things and handing them over when somebody asks.

For the grown-up

A server is a computer that waits for requests and responds to them. When a child watches something on a tablet, a server somewhere sent it. The cloud, in practice, is a building full of these.

The easy mistake

Careful here. The cloud is not a metaphor worth using with children. It is somebody else computer in a building with very good air conditioning, and the concrete version is easier to hold.

Words next to this one

Computer A machine that follows instructions very fast and never gets bored of doing the same thing again. Internet Computers talking to other computers. All of them, all over the world, all the time. Website A place on the internet where somebody keeps their things so other people can come and look. Network A group of things connected together so they can pass messages to each other.