C18 columns are used in high-performance liquid chromatography, or HPLC, to separate components of a mixture of soluble chemicals by their differences in polarity. C18 columns have chains of 18 carbons attached to silica beads on the column wall.
C18 columns are used in reversed-phase chromatography, where the column's environment is nonpolar and the mobile phase (the liquid the chemicals are dissolved in) is polar. C18 is the most commonly used reversed-phase column, and reversed-phase chromatography is the most commonly used HPLC technique. Other common types of reversed-phase columns are C8, phenyl and pure silica without any other chemical groups attached.