A mixture is a substance that is made up of different components that are not chemically combined. For example, the air is a mixture containing gaseous components.
A pure substance, on the other hand, is a substance that contains similar particles. For example, an element is a pure substance with similar atoms and a compound is a pure substance with similar molecules.
A mixture may either be homogeneous or heterogeneous.
A homogeneous mixture contains various fractions of components in a single-phase while a heterogeneous mixture contains components in different phases and different ratios.