Amazon’s FAQ describes Alexa to be an intelligent cloud-based voice service.
In other words, Alexa is for Amazon what Siri is for Apple. The difference to it is that with Amazon Alexa’s SDK and the Alexa Voice Service, it is easy to extend, integrate or deploy Alexa Skill(app) to a device which runs Amazon Alexa.
For example, most Amazon device and application have integrated Alexa. This means the My ETA skill can be installed and executed from your Fire TV stick, from your Amazon Echo or for the Amazon app on your iPhone.