Twilio provides a simple hosted API and markup language for quickly building advanced voice and SMS communications applications. The syntax and programming model are focused on making application development as close to the request/response model of web application development as possible. This allows developers to use their existing skills, infrastructure, and tools to easily add voice and SMS to their applications.
If you are new to Twilio, check out the Quickstart Guide which has a set of simple tutorials for using Twilio's APIs to build applications that receive incoming calls, make outgoing calls, send and receive SMS messages. If you need more examples, head over to the HowTo's and Example Code pages which contains the source code to a set of complete Twilio applications.