Alexa and Lex are a pair of tools built by Amazon to change the way that users interact with technology. They are platforms that allow developers to create immensely powerful conversational interfaces without having to study deep learning, natural language processing, or speech recognition.
They are part of the Amazon Web Services (AWS) group and therefore work brilliantly alongside the rest of the services, making the development process smoother and more consistent.
The main difference between Alexa and Lex is that the Alexa platform allows developers to create skills for Alexa-enabled devices, whereas Lex allows developers to create generic text or voice-based chatbots.