OH- refers to the hydroxide ion, which consists of one oxygen atom and one hydrogen atom and carries a negative charge.
A base is a substance that can accept protons (H+) or donate electrons. Hydroxide ions are often associated with bases.
An acid is a substance that can donate protons (H+). In reactions, acids tend to react with bases.
A neutralization reaction occurs when an acid reacts with a base to form water and a salt.