Define data structure for graph (abstract data structures)

Could any one please define a structure for a graph.

Could you also tell me what is the difference between
a Binary tree structure and graph structure or are they
just the same.

Example of Dynamic Binary Tree structure in pascal


PtrNode = ^Node;

Node = record
        DataArea : String[20];         {Storing string data}
        LeftPtr : PtrNode;                    {Pointer to left node}    
        RightPtr : PtrNode;                {Pointer to right node}