The brief answer is that Goods are generally something that is bought outright and exclusively, while Services are something that is bought by the use. For example, the Gas purchased at a Gas Station is a Good, while the Full Service Attendant who pumps it for you is a Service (in this case, provided either free or for a small increase in price/gallon).
Both Goods and Services are products intended to return Value for the price. However, a Good is intended to be used by the consumer and continue to return value past its purchase date -- this is why many products are advertised as "Pays for Itself!" The idea is that you will save more money in the product's lifetime than it cost to initially purchase.
A Service, on the other hand, is considered a one-time deal. If you buy a car wash, you get the car washed once. You cannot come back and claim more washes based on one purchase in this case, but you do have the right to a Valuable Result from your purchase; in this case, you expect a clean and undamaged car which will remain clean in normal operations until the next wash. Mud thrown up on your car from a passing truck is not the car wash business's concern; they provide a clean car after purchase and that's it.
You can define a Good as anything you buy and keep, a physical object, food or video games or shoes. You can define a Service as anything you pay for that is not tangible; Internet access is a service, but you are not purchasing the Internet, and you must renew your access. However, if you buy a piece of software over the Internet and download it, that is a Good provided over the Service.