The classic example of an empirical formula is HO for hydrogen peroxide, whose molecular formula is H2O2. HO could not possibly exist because one hydrogen and one oxygen atom do not have enough electrons to form a molecule. The empirical formula is formed by dividing the subscripts in a chemical formula by their lowest common factor.
The only stable form of HO that can exist is the negatively charged HO- ion. Some chemicals have the same molecular formula and empirical formula, such as water, whose formula is H2O. This is because the lowest common denominator of the subscripts in H2O is one, and dividing any number by one returns that same number. Empirical formulas were more commonly used in the past when the only available identification technique was to burn the substance and measure the products of burning. This could tell the chemist the ratios between different elements in the molecule but not the absolute number of those elements in each molecule.
Sugars and starches are known as carbohydrates because many of the common types have the same empirical formula, CH2O, which appears to contain one carbon (carbo-) and one molecule of water (-hydrate). With this simple analysis, glucose (C6H12O2) has the exact same empirical formula as a starch. However, a starch is a biopolymer that could be, for example, C1000H2000O1000.