Homeopathy is a medical science developed by Dr. Samuel Hahnemann (1755-1843), a German physician. It is based on the principle that “like cures like”. In simple words, it means that any substance, which can produce symptoms in a healthy person, can cure similar symptoms in a person who is sick. This idea is referred to as the “Law of Similars”.
An example of this principle: An onion is a substance which makes your eyes water and your nose burn. If you are having an attack of hay fever with watering eyes and a burning nose, a homeopathic remedy made from onion can relieve it.
Currently there are three thousand substances used as homeopathic remedies to treat everything from colds and cough to arthritis and cancer.
A second principle of homeopathy is that you should give the least amount of medicine necessary to evoke a healing response. This is called the “Minimum Dose”. In an effort to prevent side effects, Hahnemann began successive dilution of his medicines, to find the point at which they would be therapeutic, but not toxic.
Adapted from: http://hpathy.com