"Mag wheels" are magnesium wheels, which are strong and lighter than even aluminium, but very expensive and rarely used nowadays. Aluminium wheels would be incorrectly called "mag wheels," and wheels would also be incorrectly called "rims". "Alloy wheels" is another popular term which isn't wrong per se, except that steel is also an alloy.
An alloy is a mixture of two or more elements to form a compound with metallic properties. Usually the main constituent is metallic, but the rest don't have to be. Aluminium can be alloyed with silicon, and steel is an alloy of iron and carbon with some other elements.