These type of fins have a strap at the heal to hold the fins on. They can be made of various materials, but the more expensive ones are more comfortable due to the quality of materials. Open heal are also designed to be used with boots or wetshoes. The exception to this is Hydrosplit fins that are designed to be worn without boots.
Designed to be worn without boots. They are better suited to warmer waters and ideal for snorkeling. Again the more expensive fins are made from better material and therefore are more comfortable.
These fins are designed to provide a smoother movement and are able to transmit power through the water more efficiently. Basically that means its easier to snorkel and less tiring. This type of fin is generally very expensive.
Please note: ALWAYS put fins on and off when in the water! NEVER walk on land with them on, they will break and thats not warranty. Also, you will fall over and hurt yourself.
For more info see Mikes Diving