Without the muscular system we wouldn't be able to move, at all. We would be no different than a plastic skeleton hanging up in a biology class that is covered in a layer of dermis. Furthermore, all the major organs in our bodies like our heart and stomach are also muscles, without them we wouldn't be able to survive in the capacity that we do now at all. Our muscular system functions to give us mobility, produce heat, absorb the shock of our falls, create a shape to our body, respirate, swallow and digest our food, and pump blood through our bodies. Without it, we would not survive as an organic being.
There is no particular main structure of the muscular system though the one that often first comes to mind is our skeletal muscles - the muscles that give us mobility and strength. They are our voluntary muscles. After that people usually thinking of our cardiac muscles that circulate blood through out bodies, and finally the smooth muscles that make up our internal organs like our stomach, throat, intestines etc. Both the cardiac and smooth muscles are involuntary, we do not consciously choose to make them go, but instead they go on autonomically.