An ice shelf is a large ice sheet floating on the ocean that is still connected to the ice sheet. The characteristics of an ice shelf is that it's always calves large icebergs at the ice barrier (the boundary to the sea). The ice that forms part of the ice shelf is called shelf ice.
 
An ice sheet is considered to be an ice shelf if it is at least two meters above sea level. Usually an ice shelf is between 200 and 1,000 meters thick. The largest isshelf areas are located in Antarctica.
 
The Ross ice shelf, which covers almost the half of the Ross sea with a surface area of approx. 490,000 km². The Filchner-Ronne ice shelf, which comprises a large part of the Wedell Sea with a surface on about 450,000 km². Source: Wikipedia