They start off as a small red sphere and slowly grow to full size. When at their full size, they can spurt out short-ranged blood shots at Isaac, similar to one of Monstro's attacks. Boils decrease in size when hit, but will regenerate to their full size until fully destroyed.
Boils are nearly harmless on their own, as they can simply be killed from a distance and do not deal damage on contact. With other enemies, however, they are far more dangerous: If Isaac enters a room with a large crowd of enemies and several Boils providing covering fire, taking out enough of the Boils to give himself breathing room should be a big priority.