E means entgegen(opposite) and Z means zusammen(together)...these are german words assigned  to distinguish alkenes based on priority of substituents on both sides of the double bonds......priority is again based on atomic numbers.....e.g.
                             
                     E                                         Z
1 indicates high priority and 2 indicates low priority.....so the first molecule clearly shows that the priorities are directed opposite to each other hence assigned as E....and likewise Z for the second molecule.....(the priority is that carbon has higher atomic number than hydrogen)
important:
Isotopes: H vs D ? Since isotopes have identical atomic numbers, the mass number is used to discriminate them so D > H
						

							
hi  pranay
              E-is isomer which has higher mol.wt; groups on the opposite sides.   ex;


 here the1st is an ex, of z conformation & 2nd is E . if u like the ans plz approved it.
						

							
first give them prefference according to their atomic weight  and then lone pair..................  if same number are diagonal then e if  same number lie in  same side then they are z...............
 
approve my answer...... plz........