Does OpenStreetMap just provide GIS data? To render maps and have a browser based UI to show maps with data, would a developer need to use something like Mapnik?
OpenStreetMap provides data yes, and you'll need specific tools to make map with them. You first need to put the data in a true GIS format, which OSM is not. Think about PostGIS, an osm2pgsql tool is designed for that. Then, you'll need a rendering tool, i.e. a cartographic engine. You can use Mapnik or MapServer. Tilemill or Kosmtik can help you designing your map styles. Take a look here.