Spells are energy manipulated to do ones will. It can heal, protect, curse, hex, etc.. However it has bounderies. Magic can not affect your body ( change your hair color, gender swap, etc.. ), it can not grant you powers ( bending the elements, changing the weather ) nor transform you in to any other creature.
If you're starting out it's a good idea to read the " General Info " section found in the forums.
A spell can do many things between good luck all the way to the darker type of spells: curses. Curses are long lasting and negative ( almost always ) and spells are short lived and positive ( almost always ). As for people doing something that you wanted them to do that would probably not even be a spell or a curse but a thing called compulsion. Compulsion is a difficult task.