Gold enables you to purchase Card Packs, Service cars, purchase Garage Slots. You can purchase more Gold from the in game shop, or win Gold by winning Stars from racing and getting Gold from the prizeboard. There are also Gold Packs which are available to buy as in app purchases from the Store.
Cash also enables you to purchase Card Packs and Upgrade Cards, you can earn this in the same way as Gold via winning races, getting Stars and picking it from the Prize Boards. You can also earn Cash by selling your unwanted Cards by going into the Garage, selecting the Card and clicking the Sell button, however if you do it will be gone forever, so choose wisely what to sell. Cash Packs are also available from the Store as in app purchases.
Gold is also available via the "Earn Free Gold" button on the Main Menu, this allows players to watch Ads for Gold. Watching subsequent Ads will then result in a Gift of more Gold!