Molecular mass is for molecules, formula mass is for ionic compounds. Molecular mass and formula mass mean pretty much the same thing. The only reason we use two different terms is because molecules are covalent compounds meaning they have only non-metals so then an ionic compounds cannot be a molecule and therefore it wouldn't make sense to find the molecular mass for an ionic compound so we just call it the "formula mass" when we're talking about ionic compounds.

it's the same..molecular weight for molecular compound, formula weight for ionic compund..still, the calculation's the same

Here's what you need to know:

Molecular mass: For covalent compounds, compounds that contain ONLY non-metals such as CO2.

Formula mass: For ionic compound, compounds that contain metals and non-metals such as Na2CO3.

Metals are on the left of the periodic table and non-metals are on the right of the periodic table.

The formula mass  of a molecule is the sum of the atomic weights of the atoms in its empirical formula.
The molecular mass of a molecule is its average mass as calculated by adding together the atomic weights of the atoms in the molecular formula.
The molecular formula indicates the type and number of atoms in a molecule.
The formula mass is used to indicate the mole ratio of elements present in a compound. 
The molecular formula of glucose is C6H12O6
      The empirical formula of glucose would be CH2O.