Antihistamines block histamine, a signalling molecule the body releases during allergic reactions and uses elsewhere as a neurotransmitter. The medicines most people mean by "antihistamine" block the H1 receptor and are used for allergy symptoms and itching. A separate group blocks the H2 receptor and is used for stomach acid.
H1 antihistamines are usually split into first-generation agents, which cross into the brain readily and cause sedation, and second-generation agents, which were designed to stay largely outside the brain and cause far less drowsiness.