Although the rasterized tile sets we have discussed in this lesson are able to deliver nice-looking maps in a relatively rapid format, they can be cumbersome to keep updated and they require enormous amounts of computing resources at large map scales. To work around these challenges, a data storage format called "vector tiles" has gained popularity in the past several years. Mapbox has led development efforts on vector tiles and has shared a vector tiles specification under a Creative Commons license.
Vector tiles are exactly what you would guess: they store chunks of vector data instead of storing a map image. The idea behind vector tiles is that it is more efficient to keep data styling separate from the data coordinates and attributes. The client can use a predefined set of styling rules to draw tiles of raw vector coordinate and attribute data sent by the server. This allows the restyling of data on the fly, which is another serious limitation of rasterized tiles. Think about it: If you want to change the shade of green used to draw parks with your rasterized tiles, you must rebuild every tile containing a park. If you want to do the same thing with vector tiles, you just update your styling instructions in one place and the tiles themselves stay the same. Other display operations such as rotating the map also become easier to implement with vector tiles.
Vector tiles are designed to be small on disk, and employ a number of optimization approaches designed to reduce the amount of characters needed to store the geographic data and attributes, some of which are described in this video by Mapbox engineer Dane Springmeyer. He also introduces a product called Mapbox Studio which works with vector tiles only and is being promoted by Mapbox as a replacement for Tile Mill. The .mbtiles file format, which originally stored rasterized tiles, now only stores vector tiles when exported from Mapbox Studio.
In reality, there continue to be use cases for vector and rasterized tile formats, although it is likely that a number of organizations will see performance benefits from rebuilding some of their originally rasterized tile sets as vector tiles in the future. This is even more likely as popular commercial software packages such as ArcGIS introduce tools to work with the Mapbox vector tile specification, a strategic decision that Esri announced in a 2015 blog post. On the open source side of things, GeoServer added support for vector tiles in version 2.14, as detailed in these instructions.
Open source clients are also recognizing the staying power of vector tiles, exemplified by the VectorTile layer format built into OpenLayers 3 and plugin support for Mapbox vector tiles in Leaflet. At the time of this writing, QGIS does not natively support viewing vector tiles, although there is a relatively new plugin for this purpose.