Below are a few videos from YouTube showing just a few of the types of risks a firefighter may face when the call for help comes in.
A flashover is the near simultaneous ignition of all combustible material in an enclosed area. When certain materials are heated they undergo thermal decomposition and release flammable gases. Flashover occurs when the majority of surfaces in a space are heated to the autoignition temperature of the flammable gases (see also Flash point).
An example of flashover is when a piece of furniture is ignited in a domestic room. The fire involving the initial piece of furniture can produce a layer of hot smoke which spreads across the ceiling in the room. The hot buoyant smoke layer grows in depth, as it is bounded by the walls of the room. The radiated heat from this layer heats the surfaces of the combustible materials in the room, causing them to give off flammable gases via pyrolysis. When the surface temperatures become high enough, these gases ignite.
Video of a flashover
A backdraft is a situation which can occur when a fire's product-gases are starved of oxygen; consequently combustion slows (due to the lack of oxygen) but the combustible fuel gases (primarily CO) and smoke (primarily hydro-carbon free-radicals and particulate matter) remain at a temperature above the fire-point of the fuel gases. If oxygen is re-introduced to the fire, e.g. by opening a door (or window) to a closed room, combustion will restart, often resulting in an 'explosive' effect as the gases are heated by the combustion and expand rapidly because of the rapidly increasing temperature (see also flashover).
Video of a backdraft
Roof / Structure Collapse
While pretty self explanatory, the public tends to forget that not only are firefighters injured or killed by fire and smoke, but there is always the hazard that while the structure burns, the roof or support structures may give way causing the structure that the firefighters are working in and on to collapse. This scenario was demonstrated most prominently to the world with the collapse of the World Trade Center Twin Towers, killing thousands of firefighters, police, EMT, civilians, and other first responders. These victims died due structure collapse.
Video of a residential roof / structure collapse.
Part of being a firefighter is being prepared for the unexpected. Firefighters are called to assist in a wide variety of emergency situations where any number of possible things can go wrong. Here is just one video that shows a case where firefighters are trying to help get a victim from a vehicle. Unexpectedly, the passenger side airbag deploys and injures a firefighter.