WebGL stands for Web-based Graphics Library, and is managed by the non-profit Khronos Group. It's used in conjunction with the HTML5 <canvas> element to produce 3D graphics.
WebGL is hard to learn, because it's very low-level – it runs on Graphics Processing Units – and because it's actually a JavaScript port of OpenGL, a long-established set of APIs that game developers use. Its primary target audience is existing games developers, who've been using OpenGL for ages, so they can write games for the web platform.
Nevertheless, there are resources to help you learn WebGL – and it's not just for games; we've seen fancy graphics, visualisatons and music videos made with it. Ones I can personally recommend are:
WebGL is present in all desktop browsers (released or dev channel) except for IE10 (Microsoft has said it won't support it). On mobile, it's released in Opera Mobile 12 final on Android, Blackberry Playbook 2.0, Nokia N900, Sony Ericsson Xperia Android phones and in development builds of Firefox Mobile.