E2-Elimination of a halide and a H to form an alkene. Only can be formed if LG and H are staggered. This means it forms a trans alkene. With 3o substituent there is no competition with Sn2 , but with 2o and 1o there is competition.
Strong bases are used. Solvents are polar and can be aprotic or protic. So unlike with Sn2 H2O and ROH can be used. Small bases like NaOH used to create most subst. alkene whereas bulky bases like NaOtBu (used with solvent of HOtBu) create the least subst. alkene (in 3o, but in 2o and 1o it is always the most?).