What is botulism? How can I avoid it?

Southern Italy has recently been hit by several cases of foodborne botulism. One person died in Sardinia and two others in Calabria, where some fifteen people were also hospitalized, some in intensive care...In Calabria, sausage and broccoli sandwiches sold in a food-truck were the cause of the poisoning. An investigation is underway. But what is botulism? How can it be avoided?