There are literally thousands of different types of bacteria. Since the start of civilization (and even before) bacteria have been the leading cause of death and disease. We fight a constant battle against these microscopic critters - and usually loose those battles. The bacilli are a group of bacteria that all share some common traits.
The bacilli are all members of the same genus. This is a taxonomic classification that biologists use to categorize similar species. The genus is the next larger group from a species. The bacilli are grouped this way because they share some common traits.
For example, all of the bacilli are aerobes. This means that they require the presence of oxygen to survive. This is not the case for all bacteria - there are many species of bacteria that find oxygen to be toxic, and others that can tolerate it, but don't need to to live. The bacilli need oxygen to survive.
Bacilli are also similar in appearance. They are almost always rod-shaped microbes. They are found literally just about everywhere in nature. There is no place on Earth where you will not find at least one species of bacilli. Bacilli have the ability to form endospores when they are threatened. Endospores are a form that the bacteria takes on to protect itself for long periods of time when the local environment gets too harsh to survive. The endospore will stay that way until the bacteria is able to break free and return to its normal state.
The bacillus genus of bacteria contains a large number of bacteria that cause disease in humans. This is the group that contains anthrax. Anthrax can be used in biological warfare as it is easily able to be put in to a spore form that can be transported in powder form. Anthrax infections are treatable, but they can be deadly without prompt and proper care.
The other bacilli that is responsible for disease in humans is the bacillus cereus. This is a bacteria that is found commonly in undercooked and reheated rice. It is a foodborne illness and can make a person quite sick if they ingest b. cereus spores. Of course, there's no way to know if you've eaten them until you're sick - bummer, eh?
In addition to these two members of the bacillus genus, there are at least a couple dozen other species that do not cause disease in humans. This doesn't mean that they aren't important to the ecosystem, or that they can't infect plants or other animals. It only means that they are harmless if they infect a person.
The bacilli are an important genus of bacteria. They play an important role in modern medicine and biological warfare. Infections with the two pathological species of these bacteria are generally not a good thing, although there are antibiotics which can treat them.