YQL is a web service that allows users to access and manipulate data from APIs and web services using a SQL-like language. It acts as a "swiss army knife" for developers by providing a single interface to query, filter, join, and update data across thousands of structured data sources on the web without having to learn each individual API. The presentation encourages developers to use YQL for their projects as it handles the complexity of interacting with different data sources and lets them focus on building their applications.