Who’s On First is a global gazetteer of named places, designed to help people map and talk about places online. It’s a tool that can be used to create and manage reference data for geographic places. The data is open and freely accessible for anyone to use, and currently covers over two million different places in the world. It was created by Mapzen and is based on the GeoJSON format, which is a data format for encoding geographic data structures. Who's On First is highly modular, containing data that is organized by continent, country, region, and city.

Who's On First provides helpful identifiers for a wide range of geographic areas. These identifiers are useful for describing distinct geographical objects, as well as for identifying where data originates from and where it is located. This can be used for a wide range of purposes, such as geo-coding, real-time navigation, route calculation, search engines and more.

Some examples of capabilities that Who's On First offers are:

- Geo-Referencing: Who's On First assigns a unique identifier for each geographic area, allowing for the accurate and efficient location of locations.

- Boundaries: Who's On First also provides accurate boundaries for geographic locations, enabling applications to draw polygons around features.

- Localization: Who's On First has data localized in native language (if available) or in English, further simplifying and assisting with accurately identifying locations.

Overall, Who's On First is a powerful data set that can be used for a wide range of GIS and other applications. By providing identifiers and accurate boundaries and localization, it makes it easy to identify and accurately present locations for applications.