Hepatitis is a general term for an inflammation of the liver, whether caused by a virus, toxic substances or immunological abnormalities.
Infectious hepatitis, almost always caused by viruses, may be classified as hepatitis A, hepatitis B, 'non-A, non-B', hepatitis C, etc.
Hepatitis A is usually transmitted via food or drink infected by a sufferer or carrier of the disease.
Hepatitis B (aka serum hepatitis), and non-A, non-B hepatitis, are usually transmitted via infected blood or blood products. They may thus be passed by unsafe sexual practices, tattooing, hypodermic needles (especially among drug addicts) or blood transfusions.
Persistent infection by a virus, called chronic hepatitis, often causes cirrhosis of the liver.
Vaccines are available for some kinds of hepatitus and are required in some places for those working in situations in which they may be exposed to blood of fluids which may be contagious.
For a detailed description of the virus family, and their various medical implications, see hepatitis on Wikipedia.