Their are two forms, cis and trans versions of the isomer. The form in which the substituent groups are on the same side of the double bond is called cis:
H3C CH3 \ / C = C / \ H H
The form in which the substituents are on opposite sides of the double bond is called trans:
H3C H \ / C = C / \ H H3C
Cis and trans isomers of a substance have different physical properties. Trans isomers generally have higher boiling points and lower densities. This is because the trans isomer's molecules can line up and fit together better than the cis form.