The UTFGrid encoding scheme encodes interactivity data for a tile in a space efficient manner. It is designed to be used in browsers, e.g. for displaying tooltips when hovering over certain features of a map tile.
Since slower browsers and machines can't cope with rendering the actual polygons used to draw vectors on the map tile, we use a grid-based approach where we store the associated information for each pixel.
UTFGrid uses JSON as a container format. It is exclusively geared towards square 256x256 pixel tiles.