What root-device to use for a new EC2 instance?

I'm trying to get started with Amazon's EC2.

I have to choose an image (AMI) to start with. I've settled on using the official Ubuntu AMI, but I have two options for the root store - "instant" or "ebs". (See this page).

Being new to AWS, I'm not sure I really understand the difference. What do the two options mean, and what are the pros/cons of each?