molecular formula
Imagine you weigh out everything inside one molecule and just count the atoms by element, like reading the ingredient totals on a recipe: so many carbons, so many hydrogens, so many oxygens. The molecular formula is exactly that headcount. Ethanol, the alcohol in drinks, has the molecular formula C2H6O — two carbons, six hydrogens, one oxygen — and that single line tells you the molecule's composition and its molecular weight.
A molecular formula lists each element's symbol with a subscript for how many of those atoms appear in one molecule (a subscript of 1 is left off). By convention organic formulas usually lead with carbon, then hydrogen, then the other elements alphabetically — so glucose is written C6H12O6. What the formula does not tell you is how the atoms are connected. C2H6O could be ethanol (CH3CH2OH, a liquid you can drink in moderation) or dimethyl ether (CH3OCH3, a gas) — same headcount, totally different molecules.
In real organic chemistry the molecular formula is the starting point, not the finish line. From it you can compute the degree of unsaturation to learn how many rings and double bonds must be present, and a mass spectrometer effectively measures it for you. But because so many different structures share one formula, chemists almost always move quickly to a structural drawing that shows the actual connections.
Acetic acid (vinegar's sour ingredient) has molecular formula C2H4O2: 2 carbons, 4 hydrogens, 2 oxygens. So does glycolaldehyde, a completely different molecule — the formula alone cannot tell them apart.
One formula, several possible molecules — this is why structure matters.
A molecular formula counts atoms but hides connectivity; an empirical formula is even cruder, giving only the simplest whole-number ratio (glucose C6H12O6 has empirical formula CH2O).