Martial arts is a single system like Boxing, Jujitsu, Wing Chun ect..... that were all created (usually with no other martial art influence) from a situation in the past e.g. were under attack from an enemy so they created this art to defend themselves is a good example. MMA is made up of other martial arts or made up from pieces of other martial arts for example Jeet Kune Do, American Kempo and Vale Tudo.
One is a complex system of techniques put together and honed over the course of hundreds of years and the other is M"MA". M"MA" is a sport, nothing more.
depends. i "mix" martial arts. i have studied a few now, and i mix them as needed. but i don't compete in the sport of Mixed Martial Arts. MMA isn't real martial arts. it's watered down and controlled and monitored. there's no threat of being killed if you lose so you don't get the same psychological warfare that plays into REAL martial arts. it's a "combat sport."
short answer, yes there's a difference. is there a difference between cashews and mixed nuts?
To me there is. Martial arts implies to me a system or style with structure, testing and promotions and belts and also the particular essence of what ever that style is. On the other hand MMA implies to me a compilation of different styles and some of the main techniques of each that can be used effectively in that type of fighting. It doesn't however capture the essence of each art that it takes things from nor does it have structure, testing and promotions like those arts have either.