Dynamic sites have database backend and runs on CMS (Content Management System) in which there may be plenty of features like, you can edit it yourself on the fly using inbuilt text Editor (Like MS Word), you can have blog feature with user comments on each post, have discussion forums (will cost extra), you can display your recent Tweets dynamically, you can sell the things online after installing shopping cart feature (will cost extra) etc. And these types of websites doesn't need any coding knowledge to edit. Also you can create unlimited pages / image galleries without any extra cost.