In studying chemistry, one should learn how to read the periodic table and periodic trends in the table.. Of course, you should also know the abbreviations of each element in it so that you can identify what element is in chemical structures. Memorizing and knowing it by heart is the best way. You should also study what we call chemical nomenclature which is naming combinations of elements. Elements when combined may form ionic compounds, bonds or acids. Knowing what element reacts with another will also be necessary that's why the concept of chemical reactions must be further studied.
It is called exothermic reaction because it releases heat and light and it is called combustion reaction because it is reacting and is being oxidised by O2 to MgO.
It can also be called as oxidation reaction since Mg is oxidised to MgO.
