Recently Microsoft has released a front end framework known as Office UI Fabric.
It's a responsive, mobile-first, front-end framework, designed to make it simple to quickly create web experiences using the Office Design Language. The styling takes into account typography, color, icons, animations, responsive grid layouts and localization. Fabric uses a custom font for its iconography. This font contains glyphs that we can scale, color, and style in any way. To use the icons, combine the base ms-Icon class with a modifier class for the specific icon. Eg: <i class="ms-Icon ms-Icon--mail" ></i>
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The cdn links given by @sjkp are broken now, you can find documentation with more details here>- spo365icons.azurewebsites.net
The office fabric icons can be found http://dev.office.com/fabric/styles under the icons tab.