Sandbox
When building your application, you'll want to test in sandbox with pretend money, then pilot in production using real money.
Every API and dashboard feature is available in Sandbox.
To simulate external effects you can use special Simulation APIs, which are only available in Sandbox. They can be helpful to quickly test events that might take several hours in the real world (like receiving a wire or ACH). They're also great for testing edge cases, like returned ACHs and card refunds.
If you have a sandbox Event Subscription configured, calling these APIs will also result in the appropriate webhooks being sent to your endpoint.