Initial-scale

Since the browser tends to scale the website, this parameter sets the initial zoom level, which usually means that 1 CSS pixel is equal to 1 viewport pixel (which is not always the same as 1 physical pixel)

Maximum-scale

Setting a maximum scale means setting a maximum zoom.

User-scalable=no

This parameter removes the ability to zoom in or zoom out, and it’s even worse than maximum-scale.
 
https://developer.mozilla.org/en/docs/Mozilla/Mobile/Viewport_meta_tag
http://blog.javierusobiaga.com/stop-using-the-viewport-tag-until-you-know-ho
http://www.quirksmode.org/mobile/viewports.html