Everything worth knowing, nothing that isn't.
What it does
It maintains interconnections to operators, routes each message to the right network, manages sender IDs and compliance, and returns delivery receipts — the connectivity layer beneath your SMS API. See /answers/what-is-a-sms-gateway and /answers/what-is-a2p-monetization.
Direct vs tiered aggregation
A direct aggregator holds its own operator agreements; a tiered one buys routes from another aggregator and resells them, adding hops and reducing control over deliverability. Ask how many hops sit between the API and the operator. See /answers/what-is-grey-route-sms.
Aggregator vs CPaaS
Historically aggregators sold wholesale SMS termination; CPaaS added developer APIs and more channels. Today the terms overlap. See /answers/what-is-cpaas and /answers/how-to-choose-sms-provider.