Lightning triggers forest fires, or wildfires, but humans have been known to accidentally or intentionally set forests ablaze. Fast moving fires can kill large numbers of people and destroy vast areas of woodland and property.
Fires can be good for many ecosystems, which have evolved to cope with these events. A small fire can remove dead brush, but not kill older trees. Well intentioned fire fighting activities can allow brush to accumulate and leave large amounts of fuel for a future large and potentially unmanageable fire.