What is Z and E isomers?

When each carbon of a $\ce{C=C}$ double bond carries two different atoms or groups, two stereoisomers exist, because the double bond does not rotate. On each carbon, the group whose atom bonded to it has the larger atomic number has priority. If the two priority groups are on the same side of the double bond, the isomer is Z; if they are on opposite sides, it is E.