Short Definition
Homeopathy is the art and the science of healing the sick by using substances capable of causing the same symptoms, syndromes and conditions when administered to healthy people. Homeopathy aims to aid and stimulate the body's own defense and immune processes. Homeopathic medicines are derived from a variety of plants, animal materials and minerals.

Expanded Information
Therapeutic system of medicine that is based on the principle of similars- like cures like - which means that a substance that can cause certain symptoms in a healthy person can cure similar symptoms in an unhealthy person. Any substance may be considered a homeopathic medicine if it has known "homeopathic provings" and/or known effects which mimic the symptoms, syndromes or conditions which it is administered to treat, and is manufactured according to the specifications of the Homœopathic Pharmacopœia of the United States (HPUS).

Official homeopathic drugs are those that have been monographed and accepted for inclusion in the HPUS.

Central to all homeopathy is the determination of the effect of substances on healthy volunteers and the use of the developed "drug picture" by the consumer and/or trained health care practitioners according to the homeopathic principle of similia similibus curentur - Let likes be cured by Likes.

Historically, homeopathy has been practiced by medical doctors, and has been used for self-care by the general public. The issuance of The Homœopathic Domestic Physician by Constantine Hering, M.D., (1835) opened this health care modality to the public.

Homeopathy is an ideal therapeutic medium for self-medication of symptoms usually associated with self-limiting conditions since the selection of the proper remedy for the case is dependent on the symptoms that the body exhibits in its reaction to the illness.

In the use of homeopathy for conditions which are other than self-limiting, the consumer is advised to use the services of a health care provider.

How it works:

How these incredibly minute doses can have such resonance in our bodies is a little too technical for science quite yet, though science is finding more and more applications for energetically diluted substances in and out of medicine from engine additives to experiments in chemotherapy.

But the way we see it is this: Our bodies have incredibly sensitive receptors which trigger adjustments and responses to even the slightest changes and dangers in our environment. Changes which thanks to these receptors, we usually know nothing about. The body adjusts to every temperature change, to chemicals in the air and in our food, to fluorescent lights, germs and bacteria present in every breath we take, it responds to bitterness, dryness, acidity, .... Only when these aggravants are in excess or our reactions are too burdened do we ever know what changes our systems have made to keep us comfortable and safe. This is not news to those of us who know the healing powers of homeopathy's extremely minute doses. In response to the slightest hint that something is present which can cause a certain set of symptoms, the body sets out to heal itself.

Source: http://www.homeopathy.org/