SVG is a vector based web graphic format. SVG files are very small, but readily scale to any size without losing quality.
To use these SVG versions of the logo, copy and paste the markup into the source file of the page you are working on. This markup has been optimized for accessibility. You can customize the styles, but do not remove the <title> tag.
PNG files are raster based images that are supported by a large range of software. They will lose quality if scaled up substantially.