The superfamily Plesiosauroidea represents one of the two major traditional divisions of plesiosaurs, the group that typically have long necks. The other major group (superfamily) of plesiosaurs is the Pliosauroidea (often just called pliosaurs). This simple division of plesiosaurs into two superfamilies has been called into question in recent studies and the picture is probably more complicated, with at least some true plesiosaurs being located outside of these two clades. Within the Plesiosauroidea, there are a number of families including the long-necked Plesiosauridae, Cryptoclididae,Elasmosauridae, and there is an increasing amount of evidence indicating that the short-necked Polycotylidae also originated from this group.
In popular literature the superfamily Plesiosauroidea, and also the larger group Plesiosauria, are both sometimes referred to as 'plesiosaurs', and this can sometimes result in confusion. See my article on different concepts of plesiosaur for the full story. Throughout this site I use the informal term 'plesiosaur' only when referring to the Plesiosauria, and use the word 'plesiosauroid' for the Plesiosauroidea.