Alexa provides a set of built-in capabilities, referred to as skills. For example, Alexa’s abilities include playing music from multiple providers, answering questions, providing weather forecasts, and querying Wikipedia. Check this doc for more information.
The Alexa Skills Kit lets you teach Alexa new skills. Alexa skills are like apps. You can enable and disable skills, in the same way that you install and uninstall apps on your smart phone or tablet. Skills are voice-driven Alexa capabilities. You can add Alexa skills to your Echo to bring products and services to life. Customers can access these new abilities by asking Alexa questions or making requests. You can build skills that provide users with many different types of abilities. For example, a skill might do any one of the following: