The four isomers of butene (C4H8) are but-1-ene, (Z)-but-2-ene, (E)-but-2-ene and 2-methylpropene. The common names of these isomer are, respectively, α-butylene, cis-β-butylene, trans-β-butylene and isobutylene, notes Wikipedia.
Isomers are molecules that share the same chemical formula but have different structures of atoms and bonds. The isomer α-butylene has two double-bonded carbon atoms, one of which has single bonds to two hydrogen atoms, and the other has single bonds to one hydrogen atom and one CH2 ion. This CH2 ion has a single bond to CH3. In cis-β-butylene, each carbon atom has a single bond to one hydrogen atom. One carbon has a single bond to H3C, and the other has a single bond to CH3.