Free-loading is taking something that isn’t yours because you can. A free-loader will go to someone they don’t knows buffet. He will ride the bus without buying a ticket or steal a cable signal.
A free-rider on the other hand will buy groceries without comparison shopping because the market has priced most items for careful shoppers. She will follow the crowd when going to an event not bothering to plan in advance.
The difference: Free-loading gives a benefit to the free-loader but there is a cost to the people taken advantage of. Free-riding is an advantage to the free-rider without imposing a cost on others or society.