Dashicons are shipped with WordPress and made available to developers looking to use custom icons in their plugins and themes.
It's a great little resource that makes stuff like icons for custom post types much simpler.
Currently, Dashicons are an icon font. However, more and more folks (myself included with the front-end of Post Status) are moving from font icons to SVG. Chris Coyier has the post to read on why SVG is pretty much always the superior option to icon fonts.
There's a small team that works on Dashicons — keeping them updated with new icons, updating current ones, etc — and right now they are discussing how to switch from an icon font to SVG. They had a meeting just yesterday, where developer Kelly Dwan posted a Github repo with an example implementation of Dashicons as SVG. You can see a sample view of the WP Admin with SVG icons here.
This is a great move. Kelly tells me that they're now doing “workflow experimentation” before an official patch, in order to define “what grunt tasks to use, how to get browser support, etc.”
The WordPress admin has evolved so much over the years, and while it seems the look of the admin is pretty settled now, I'm loving the progress on the implementation. Between this, some efforts to reorganize the CSS, and (what I hope is inevitable) a future WP API driven admin, it's getting some serious love.