In addition to the basic "Americans use hood and Brits use bonnet but it's the same thing", you can look at the origins for the term and see that they both also describe very similar pieces of headgear:
A hood is a cold weather cover for your head...usually nowadays we would say it is attached to a jacket or coat, but it used to be more common for it to be a completely separate piece of clothing. It covers the back of your head completely and usually comes forward a little bit, shielding the face without covering it.
A bonnet is an old fashioned type of hat that women used to wear, which covers the back of the head and usually comes forward, shielding the face without covering it.
In old fashioned cars, the shape of the hood/bonnet actually slightly resembled a hood/bonnet (clothing). So it's no surprise that people picked those words to describe it.