Scalable vector graphics (SVG) are resolution independent images and look very good even at non-native resolutions (e.g. Retina displays, or when zoomed in or out).
The majority of browsers now support SVG [http://caniuse.com/#feat=svg]. The only downside is that IE <= 8 and Android <= 2.x can not display these images. For these cases it is suggested to have a fall-back image in standard format such JPEG or PNG.
The sphinx's PDF builder also does not support SVG, but the following synthax directs Sphinx to look for the most appropriate format among those available:
.. figure:: _images/image.*
It means that for the case where _images contains image.pdf and image.svg, the HTML builder will then use image.svg and the PDF builder image.pdf.