The API covers the core objects you'd expect: customers and locations, jobs and job statuses, invoices and payments, technicians, and Pricebook items. Webhooks exist for several of these, so a job getting marked complete or an invoice getting paid can trigger an outside system in near real time instead of waiting for a scheduled export to run.
The Marketplace sits alongside the raw API as ServiceTitan's own directory of pre-built integrations, mostly covering common needs like accounting sync, marketing attribution, and payment processing. It's a solid starting point for standard needs, but it's a fixed set of features someone else built; it doesn't flex to a business's specific rules the way a custom integration against the API does.