SN1 – a stepwise reaction, which relies heavily on the stability of the carbocation intermediate (3°>2°>>1°). SN2 – a concerted reaction which occurs via a backside attack and relies on the steric hinderance of the molecule (1°>2°>3>>°).
SN1 reactions occur with strong nucleophiles (negatively charged), in the presence of a polar protic solvent, and often create a racemic product. SN2 reactions, however, take place with weak nucleophiles (neutral), in polar aprotic solvents, and result in inverted products.