A TT (time trial) or triathlon helmet has a distinctive teardrop shape with a long tail that extends behind your head. This design smooths airflow over your back when you're in a low, tucked aero position, reducing drag more than any other helmet type. It's purpose-built for time trials and triathlon bike legs, where every second counts.
The key catch is that these helmets only deliver their aero benefit if you maintain that tucked position. If you're frequently looking around or sitting up, the long tail can actually catch the wind and create drag. They also have minimal ventilation, so your head will run significantly hotter than with a standard road helmet.
A TT/triathlon helmet makes sense if you're competing in events where aerodynamics matter and you can hold your position. For training rides, casual events, or road races with frequent position changes, you're better off with a standard road or aero road helmet.