Defiants were different to other fighter planes because their guns (four Machine guns) were in a rotating turret and could fire in any direction. They had two crew: one man was the pilot and the other aimed the guns.
Defiants were made to shoot down enemy bomber aircraft. When they were used during the Battle of Britain, they were good at this. But they were slower and could not turn as well as German fighter planes, like the Messerschmitt Bf 109. A lot of Defiants were shot down by German fighters.
After the Battle of Britain, the British only flew Defiants at night. In this way they could attack the German bombers that also flew at night, and avoid the German fighters because they only flew during the day.