A column is a pipe packed with material through which vapor travels. On the material used to pack the column, some of the vapor condenses into a liquid. The liquid then heats up and turns into a vapor at which time it travels a bit further up the column. When two compounds such as water and ethanol are both traveling through the column, the one with the lower boiling point (ethanol) travels faster through the column.
There are several materials that can be used to pack a column. One of the characteristics of a good packing material is a large surface area per given volume. Common materials used as column packing include: