Relative atomic mass
How heavy different atoms are compared to the mass of an atom of carbon-12.
Carbon-12 has an atomic mass of exactly 12.
Relative atomic mass is usually just the same as the mass number of the element.
The bigger number on top in the periodic table is the relative atomic mass.
Relative Formula Mass
All the relative formula masses added together.
Eg, MgCl = Mg(24) + Cl(35.5) = 59.5
Different atomic forms of the same element.