Sources send events in. A website, mobile app, or backend server sends structured events, a signup, a purchase, a support ticket closed, into Segment through its SDKs or server-side API, each one tied to a specific user or anonymous visitor.
Segment unifies and forwards it. Those events get stitched into one profile per customer, then routed out to whichever destinations you've connected, an analytics tool, an ad platform, a CRM, a warehouse, without you writing a separate integration for each one.
It's not a messaging product. Segment and Twilio's SMS and voice APIs are separate tools under the same company. Segment deals in data and audiences, not sending texts or calls directly, so pairing the two, texting someone based on something Segment just learned, is still something that has to be built.