What is the difference between a mobile friendly site and a normal desktop type site?
A mobile site is responsive and retina ready.

Responsive; This means a web page can automatically resize to any size of browser. It also resizes in an attractive way with text and images re-organising itself so that the content remains attractive and considerate to smaller screens. Text automatically assumes a nice size and the gaps between content is consistent. The way it works is when a user visits the site a small script of code is used to identify the size of browser being used. This indicates the of device type. The site then loads accordingly.

Retina ready; This is a phrase first coined by Apple when the iPhone 4 arrived. The screen size of the iPhone 4 is 3 and a half inches, and its resolution is 960x 640 pixels. At this resolution, at a normal viewing distance, the human eye finds it hard to make out individual pixels, so this technology was branded as retina display.For example, a MacBook with 15 inch display has a pixel resolution of 220 pixels per inch. Older monitors use a resolution of 72 pixels per inch. Without getting very technical, this basically means that Retina display devices show more pixels in every square inch, this makes images a lot sharper on the eye. A single pixel is the smallest unit used on a screen. So, the bottom line is that for Retina devices, higher resolution images are absolutely required. If 72 PPI images are used for a Retina display they will be much lower quality and wont be as sharp or as vivid as 220 PPI images.