In The Empire Strikes Back, when Lando sells out Han and the good guys to the Empire he arranges a meeting where he leads Han, Leia, and Chewbacca into an ambush. When the doors open and Han sees who is in the room, he reacts immediately and shoots at Vader. Why does he do this? Is Vader so well known as a bad guy or is it because he recognizes him from the few seconds he saw him escaping the death star?
Most rebels would recognize Darth Vader in the same way that many WWII soldiers would have recognized Hitler, even though they never saw him. Photos and verbal descriptions would be enough. But Han had seen Vader fighting Obi Wan on the Death Star. Leia very likely would have explained who the giant guy in all black armor was in the time since.
As to why would Han's reaction be to draw and shoot as fast as he did, instinct. Despite what George Lucas would have you believe with his silly retcon, Han Solo was happy to shoot first and dispatch a dangerous enemy if the opportunity presented itself. When suddenly faced with such an immediate and deadly threat, Han's first instinct was to shoot.