Because you're limiting yourself to a single sentence. How about this: "The internet is essentially an expansion of the concept of a telegraph - it is already possible to pass any type of information between two distant people almost instantly. The internet combines this system of information transfer with machines that know how to respond to messages without human intervention - under the condition that the messages follow a certain set of rules. By defining these rules in advance, the telegraph system, which up to now only transferred individual letters, can be co-opted to send things such as pictures and films. The machine can tell who is speaking with it, and perform tasks for that particular person upon request."
This does not sound stupid - it might take some explaining, but anyone with a brain between their ears can understand the basic concept of fast information transfer. They might not think of all of the possible uses of such an invention right away, but I bet that they would easily understand the uses if they were explained in terms of things these people already know.