It looks suspiciously like networks divided into the seven OSI functional layers or data-center diagrams drawn by people with so weak a sense of space they put the storage 100 miles from the transaction processing monitor.
The interesting thing is, you can rent every layer, every stratum of data-center function from someone different, if you like, no matter how far apart they are.
If you look at any data-center diagram or illustration of OSI, you see the bottom is a physical layer – the wires that run from your data center to the cloud-service's data center.
You rent them from telcos, which traditionally operate exactly as cloud providers do, though in a less evolved way. (In the Telco language, "cloud" means "don't worry your pretty little head about all that technical stuff. Just plug into the wall, pay us a lot of money and we'll take care of everything. Eventually.)
You have to do the Layer 2 part yourself – packing your odd-tasting foreign data into packets other networks are willing to take with them and apps are willing to taste.
That's the only part, though.
The Layer 3 – Switching and Routing – also comes from telcos, as does Transport in Layer 4. (Both are actually Transport; it's just that Transport 4 is a little smarter than Transport 3 and there's more money in making hardware for Transport 3, so it's treated by vendors as being really important.
Layer 5 – Session – is what cloud platform software is all about. It's a broker that sets up connections between apps and databases, apps and other apps, apps and networks, deals with their conflicts and fights, divorces them when necessary, and kills or drives them off the cloud when it's time for the end user using them to go.
IAAS services like Amazon's EC2 live here. It lets you play in the data center and expects you to do something useful while you're at it, which is surprisingly complex and difficult for technology that's supposed to be easy.
Talking the apps into and out of all those things is tricky, but cloud providers don't do it any differently than you do. They stuff so many APIs, middleware and message-exchanges into a box that it squirms, like stuffing a shoebox past the bursting point with live mice that try harder to get out as you try to stuff more of them in.
The cloud providers try not to let you see that, though. It's kind of industrial and ugly compared to the shine and the fog.