Often I'm asked to explain BTC to someone and occasionally I come up with something stellar that really nails it, but most times it's a rambling incoherent mess.
What I would like to see is a short and simple explanation of what BTC is that is suitable for someone non-technical to understand.
You should explain bitcoin as it is. The simpler the better because not all people can understand such complicated things. Tell them that it is a currency running digitally and there is no entity that is controlling it. By the time that they ask questions, it means they are interested and that is the time you should be answering them one by one.