Let me try to answer:
"ما" and "ماذا" are interrogatives with the same meaning "What ?".
But "ماذا" can only be used in relation to a verb - while "ما" can be used both before a verb or a noun/pronoun.
Here are some examples, picked from the other answers to your question:
Before a noun: only "ما"
ما الأمر؟ = what's happen?
ما سبب زيارتك ؟what is the reason for you visit ?
ما هو لونك المفضل ؟؟what is your favorite color ?
ma houa ra'iok? = what is ur point of view?
Before a verb ("ماذا" always possible, "ما" also possible most of the time)
ماذا تفعل؟ = what are you doing? // do not confuse with
ماذا تفعل غدا؟ what will you do tomorrow ?
ماذا قال لك الاستاذ ؟ = what did the teacher tell you ?
؟ماذا تحمل بيدك ؟ = what are you holding ?
بماذا تحلم بما تحلم ؟؟ = what do you dream of ? see that here both can be used .
Note how you can switch between ""ماذا" and a phrase like "ما هو / ما اللذي" :
ماذا يحدث هنا ؟ = ما الذي يحذث هنا ؟