This document discusses the Twilio API for building text messaging capabilities into web applications. It introduces Twilio and its features for SMS and voice capabilities. It also provides instructions on getting started, including creating an account, getting a verified phone number, writing scripts to make calls and send SMS, and using Twilio with different programming languages. Key features of Twilio like global reach, logs and analytics, and the Twilio Client are also covered. The document concludes with a comparison of Twilio to other APIs like Tropo and Nexmo.