The Alexa Skills Kit allows developers to create custom voice user interfaces for various skill types, including games, music, and food. To build an Alexa skill, developers need an Amazon developer account, Alexa Skills Kit Command Line Interface (ASK CLI), Node.js and npm, Git, and AWS IAM user. The skill can be created using ASK CLI or the Alexa Developer Console, with options for hosting on Alexa-hosted skills, AWS CloudFormation, AWS Lambda, or self-hosting. Once created, the skill can be edited to add custom interaction models, intents, slots, and sample utterances, which determine how the skill responds to user input. The skill can also be tested using the Alexa Simulator, with options for saving and using sample utterances and commands.