Upton Sinclair used to say- "It is difficult to get a man to understand something, when his salary depends upon his not understanding it!"
I think - "It is difficult to get an on-premise CEO to understand something, when his maintenance revenue depends upon his not understanding it!", call it the Cloud Computing Sinclair Corollary.
For the rest of us that don't have that problem, its rather easy - an application that works on the internet without requiring you to install lots of software or buy hardware.
If you need further help, here is a video (hat tip Vinnie):
Next time you hear a CEO explain how he's finding it difficult to differentiate between decades of on-premise software and hardware, and cloud computing - give him a hug. He just can't understand - the maintenance revenue prevents it.