Cheetah are not classed as 'big cats'. Big cats have the ability to roar - cheetahs have instead retained the ability to purr, just like your domestic cat at home.
Fact about Cheetahs
Cheetahs have black "tear marks" that run down their face from the corner of their eyes to their mouth. It is believed that the "tear marks" aid in hunting by helping to keep the sun out of their eyes.