I can attest to this.
You pay money into an account, use a library (most languages are supported), and are charged per text (I think one cent for normal SMS?)
Messages are received by specifying a URL end point to twilio it will send the data to.
I liked the good documentation, and the free demo period. You get access to everything, just with an "ad" at the end of everything you send.
(I've built a few apps with this and had no issues)