With the Twilio API for WhatsApp, developers can send messages to WhatsApp numbers, including plain text and files like images, audio, and PDFs up to 5MB. To get started, a Twilio account is required along with Ruby and Bundler installed, as well as the WhatsApp Sandbox Channel activated. The Twilio-ruby gem needs to be added to a new Gemfile and installed using Bundler. A sample code in `app.rb` can be used to send a message from the WhatsApp Sandbox number to the developer's own WhatsApp account, replacing placeholders with actual details such as account SID and auth token. Running this code results in receiving the message in the WhatsApp app. The Twilio API also supports sending media messages, including images, by adding a `media_url` parameter to the message creation call, which points to an online image file with matching content type header.