Screamer = lots of little bangs (say 4 in a 360' crank cycle)), Big Bang = less bangs but the bang is way bigger (like 2 in a 180' crank cycle)

Imagine a 4cyl motor, a screamer fires all cylnders evenly accross the crankshaft (4cyl roadbikes)
Imagine a 4cyl motor, A big bang fires 2 cylinders close together, turns the crank 180' and fires the other 2 cylinders close together. Kinda like a Ducati L2 but with a 4cyl where 2cyl are pair'd to act like 1....1 big one
Its said a screamer aplys constant drive to the wheels, a big bang provides a longer rest period between pulses and that is easier on tyres, wheel spin is easier to control and corner drive is less violent.....in a nut shell