Amphotericin is one of two anti-fungal antibiotics produced by the bacteria Streptomyces nodosus:
Amphotericin A: This is the less active form of the antibiotic. It very toxic and is not used clinically but is sometimes used in cell biology research. It is produced alongside amphotericin B and must be removed to create a medically-usable antibiotic drug.
Amphotericin B: This is the active form; it is a potent drug given intravenously to treat systemic fungal infections that are resistant to other treatments. It is most often used to treat candidiasis and histoplasmosis. At low doses, it prevents fungal growth; at higher doses, it actively kills fungi. Its chemical formula is C47H73NO17.
Amphotericins work by binding to a lipid called ergosterol, which is an important component of the fungal cell walls; when the ergosterol molecule binds to the amphotericin molecule a hole forms in the cell wall that causes the contents to leak out. The fungal species it works against include Candida, Cryptococcus, Histoplasma, Blastomyces, Coccidioides, and Aspergillus.
Unfortunately, amphotericin B is significantly toxic and can cause nasty side effects such as kidney damage, liver damage, anemia, nausea, chills, rashes, muscle cramps/weakness, vomiting, and headaches. It also causes the body to lose potassium and magnesium. Patients (particularly children) are sometimes pre-dosed with acetaminophen (Tylenol), diphenydramine (Benadryl), hydrocortisone, and demerol to lessen the severity of the side effects. People who have had to take it have given it the nickname "amphoterrible".
Some of the information in this writeup was taken from the science dictionary at http://biotech.icmb.utexas.edu/; the rest was gleaned from http://www.swosu.edu/~longs/phcl/antifung.htm and http://www.vh.org/Patients/IHB/Peds/PBMT/42MedicationInstruction.html