From Their Site

What Problem Does Twilio Solve?

We're always building web applications, and sometimes we want those apps to be able to interact with phone callers. Maybe we want a customer to be able to call in and get information, or maybe we need to coordinate our employees more efficiently. Before Twilio, you would have had to learn some foreign telecom programming languages, or set up an entire stack of PBX software to do this. At which point, you'd say "aw, forget it!" Twilio lets you use your existing web development skills, existing code, existing servers, existing databases and existing karma to solve these problems quickly and reliably. We provide the infrastructure, you provide the business logic via HTTP, and together we rule the world.