The API
A casino API is the content integration layer: catalog, game launch and wallet transactions. It assumes you already have — or are building — a platform with players, balances and a lobby.
The platform
A casino platform is the operational core: player accounts, wallet, bonuses, admin panel and reporting, with APIs connecting content and payments to it.
Which do you need?
If you have a platform and want content, you need an API. If you are starting from zero, you need a platform — which will itself use APIs for content and payments. If you are unsure, the scoping conversation usually answers it within one call.