When the selected event is triggered, the underlying feature attributes are shown below.
UTFGrids can be used to output highly optimized feature "hit grids." The UTFGrid encoding scheme encodes interactivity data for a tile in a space efficient manner. It is designed to be used in browsers for interactive features like displaying tooltips without having to hit the server for an "info query."
See the utfgrid.js source for detail on using UTFGrids in OpenLayers. For more info, view the UTFGrid Specification.