A homogeneous mixture is the one which has the same uniform appearance and composition while heterogeneous mixture is the one which has visibly different substances and they are really unmixed.
In the light of above definitions:
A mixture of oil and water is heterogeneous because oil and water do not mixed completely with one another.
Air is a homogeneous mixture of the gaseous substances like nitrogen, oxygen, and smaller amounts of other substances. But a polluted air is heterogeneous because it has particles which do not get mixed with gases.
Cereal in milk is heterogeneous because they do not get mixed too.
A glass of tea is homogeneous because tea, sugar and milk all are mixed in water completely and make a uniform composition.
