HO- is a very common nucleophile. Remember a nucleophile is a species that desires to donate its electron density to another compound. In an S_N2 reaction, the electron density of the HO- molecule (shown in red) will attack the backside of a carbon-X bond (S_N2) or a carbocation (S_N1). HO- is also a strong base so it can remove a proton. (E1 or E2).

Methyl chloride can be involved in S_N2 reactions. A nucleophile (HO- , for example) will attack the backside of the carbon connected to the Cl-, and the Cl- will be forced out as a leaving group. Notice that the area of high electron density on the nucleophile (red area) will attack the area of low electron density on the methyl chloride molecule (blue area). This backside attack will cause an inversion in the stereochemistry.

Anti-periplanar geometry is required in E2 reactions. That is, an elimination will occur (or a double bond will be formed) only if the base attacks the hydrogen that is in the same plane as the leaving group. This antiperiplanar geometry allows the orbital to line up as the pi bond is forming.