The M8 Greyhound is a light armored car, armed with a 37mm cannon for both infantry support and light anti-armor roles, and an M1919A4 .30 caliber machine gun for infantry suppression.
Light armored cars are mostly infantry support units, that help stand against heavy machine-guns and other light support vehicles; the M8 Greyhound is a perfect choice for early game raids, with it's phenomenal speed, you'll be knocking on the enemy's supplies before he has a chance to realize it.
As an infantry support unit it's very proficient: this car eats dug-in MGs for breakfast and has very good suppression fire. But when it comes to facing other light vehicles you must rely on numbers, and any light tanks will stop you dead on your wheels. It's better to retreat and find a weak spot on the enemy's defenses whenever you get to face tanks.
Try not to invest too much of your command into more than a handful of these light vehicles, as later on they will become useless against enemy tanks, which far out-range them and have superior armor and firepower.