Anytime a nest is disturbed workers come out ready to defend the colony. Fortunately, most ant species are only able to bite. During a bite the ants use their mandible to pinch the human skin.
The most feared of all ants is the red imported fire ant because they do both, bite and sting. Fire ants grab the skin with the mandibles and use it as leverage to insert the stinger multiple times. The venom injected by the ovipositor causes irritation and burning sensation.
Fire ant bites/stings are life-threating to a small percentage of the population (less than 1%). Certain proteins in the venom can cause anaphylactic shock, a severe allergic reaction that can be deadly without intervention. In response to the sting the body releases a flood of chemicals that cause you to go into shock. An injection of epinephrine is required to halt this event.