Google Webmaster Tools Fetch vs Fetch And Render?

Google Webmaster Tools Fetch vs Fetch And Render?

Here is something I have been wondering about. On Google Webmaster Tools there are two options.

One is Fetch and the other is Fetch And Render.

Can anyone please explain to me what the difference is between these two options and when I would use each one?

Up until now I have only used Fetch but I have been curious about Fetch And Render.

There is also a dropdown menu with the following choices:

• Desktop
  
• Mobile: Smartphone
  
• Mobile: XHTML/ WML
  
• Mobile: cHTML
  

What are all these options for? I just leave it on Desktop because that is where it is when I access it.