Scalable Vector Graphics is a vector graphics format that describes images with XML. It's been around since 1999 and is supported by all major browsers these days. Unfortunately, Internet Explorer has been a laggard and provides only limited support since Version 9. Vector images can be rendered in any size without becoming fuzzy. This means you can render the same image on a large retina display or a small mobile phone, and it will look great in both cases.
SVG images are made up of shapes you can create from scratch using paths, or put together from basic shapes defined in the standard, for example, a line or a circle. The format itself represents shapes with XML elements and some attributes.
As such, SVG code is just a bunch of text you can edit manually, inspect with your browser's normal debugging tools, and compress with standard text compression algorithms. Being text based also means you can use d3.js to create an image in your browser, then copy and paste the resulting XML to...