I am very confused about how index options/futures work. They don't seem to have any tangible underlying asset like stocks, currency, Corn, wheat, etc.--just numbers on an index.

For example, what happens if I buy an S&P e-mini contract and let it expire? Do I don't get 5000 bushels of S&P stocks?
Or if I buy an index call option, could it be excercised? It's just all very confusing. Can anyone shed some light on the subject???