In the modern web development it's common practice to use fonts to display icons. Since fonts are vectors, they are resolution independent and can be easily colored through CSS, just to name a few advantages compared to bitmap images etc. Ionicons was created by the same team that Ionic Framework was created by and can be used in any project since they are 100% free and open source. MIT Licensed.

Ionicons can be used on their own or with Ionics CSS components when they have certain styles according to the parent elements.

The homepage and list of the icons can be found here: