Do I use , , or for SVG files?

I can recommend the SVG Primer (published by the W3C), which covers this topic: http://www.w3.org/Graphics/SVG/IG/resources/svgprimer.html#SVG_in_HTML

If you use <object> then you get raster fallback for free*:

<object data="https://stackoverflow.com/questions/4476526/your.svg" type="image/svg+xml">
  <img src="https://stackoverflow.com/questions/4476526/yourfallback.jpg" />
</object>

*) Well, not quite for free, because some browsers download both resources, see Larry’s suggestion below for how to get around that.

2014 update:

• If you want a non-interactive svg, use <img> with script fallbacks
  to png version (for older IE and android < 3). One clean and simple
  way to do that:

  <img src="https://stackoverflow.com/questions/4476526/your.svg" onerror="this.src="your.png"">.

  This will behave much like a GIF image, and if your browser supports declarative animations (SMIL) then those will play.

• If you want an interactive svg, use either <iframe> or <object>.

• If you need to provide older browsers the ability to use an svg plugin, then use <embed>.

• For svg in css background-image and similar properties, modernizr is one choice for switching to fallback images, another is depending on multiple backgrounds to do it automatically:

  div {
      background-image: url(fallback.png);
      background-image: url(your.svg), none;
  }
  

  Note: the multiple backgrounds strategy doesn’t work on Android 2.3 because it supports multiple backgrounds but not svg.

An additional good read is this blogpost on svg fallbacks.