YQL (Yahoo! Query Language) allows users to query, filter, and join data across different web services and data sources using an SQL-like syntax. It supports various data formats and provides tools for developers to build applications using YQL and external data sources. YQL tables can be created, tested, and stored on Yahoo's cloud servers for application development and data can be accessed through a REST API or YUI module. It has globally distributed servers for fast performance and provides options for public, authenticated, and personalized data access.