Exclusivity

Only Dreamstime, Fotolia and Istockphoto offer exclusivity for some or all of your pictures.

Exclusivity generally means if you have opted in you cannot sell your pictures on another stock site or on your own website "royalty free".

Being exclusive on the most known example iStockphoto.se means you can still sell pictures on other sites but not the ones you have uploaded to iStock. They will reward you for the limitation though. Your share will increase from the current 20% up to 40% (depending on volume) plus that they will expose your pictures on the front page under the exclusivity tab. My experience is that by going exclusive at least doubles the number of sales.

But!...
You can still sell the pictures to companies and newspapers or any other customer. You have not given up any legal rights to your pictures.