Anthrax is a gram-positive bacterium, Bacillus anthracis, that produces a toxin, which is what kills you. The reason they are so problematic, is they have the ability to form spores, which protect their genetic material, and last for a very long time.
Most of the time, if you have a cutaneous infection (infected through a cut in the skin), then antibiotics will work just fine.
However, the other two types of infection, gastrointestinal and respiratory, are usually fatal.