Everything worth knowing, nothing that isn't.
What it provides
Channel APIs, operator routing, sender-identity and compliance handling, delivery receipts, and inbound message routing — plus, on fuller platforms, segmentation, journeys and analytics. See /answers/what-is-cpaas and /answers/what-is-a-notification-api.
Platform vs gateway vs CPaaS
A gateway is the connection layer; a CPaaS exposes channels as developer APIs; a full messaging platform may add engagement tooling on top. The terms overlap and many vendors span all three. See /answers/what-is-a-sms-gateway and /answers/what-is-customer-engagement-platform.
What matters underneath
Whatever tooling sits on top, the value depends on route quality and deliverability beneath it — direct routes, real delivery receipts, throughput and an SLA. See /answers/how-to-choose-cpaas-provider and /products/omnichannel.