The <canvas> tag was initially introduced by Apple into WebKit for use in Dashboard widgets, rather than for web use. Since then, Opera and Firefox have also gained implementations. Internet Explorer 9 is expected to implement it, but IE 8 and earlier don't.
One of the aims of the HTML 5 standard is that third parties should be able to implement HTML 5 on legacy browsers. Internet Explorer doesn't provide its own canvas element, but it does provide vector-drawing functions in the form of Microsoft's Vector Markup Language (VML). The Explorer Canvas script uses this capability to implement canvas support on top of the native VML support.