The hydrocarbons in the different fractions differ from each other because their molecules are different sizes. Most of them are members of the alkane homologous series.
Fractions that come off near the top, such as gases and petrol, have short molecules with fewer carbon and hydrogen atoms. Fractions near the bottom, such as fuel oil and bitumen, have long molecules with many more carbon and hydrogen atoms.
As the molecules get longer, the boiling point increases, because more energy is needed to separate the larger molecules. Longer molecules are also harder to ignite, so short molecules are easier to ignite.
Viscosity is how easily a liquid flows. Short molecules give a runny liquid with low viscosity. Long molecules give a thick, sticky liquid that is more viscous, such as bitumen.