C18 is octyl decyl silane (ODS), that contains 18 carbons bound to the silica. So C18 have more carbons & a longer carbon chain than usually C8 (contain 8 carbons) or C4 (contain 4 carbons).
Having extra carbons chain, C18 has a larger surface area and more denser so that the mobile phase has to travel across more.
This leads to more interaction time between the bonded phase of the column and the elutes traveling through a mobile phase. As a result, the sample (Analyst) elutes more slowly and gives more separation.
C18 column is based on hydrophobic interaction mechanism.
C18s are good for long-chain fatty acids and complex molecules.
Examples of C18 Column Used in Pharmaceutical Widely:
Zorbax C18 by Agilent
Allure® C18 by Restek
Symmetry® C18 by Waters
Superspher® 100 RP-18e by Merck
Hypersil® ODS by Thermo/Hypersil
Luna® C18 by Phenomenex
Related: Number of theoretical plates (N), Resolution (RS), Column Storage, Column Cleaning and Regeneration of HPL column.
Other hand, C8 (also known as octadecyl) is better for shorter retention time (RT) and delivers sharper peaks in HPLC. C8 column types are mainly used in small organic compounds.
Related: What is Stationary phase in HPLC ?
Examples of C8 Columns Used in Pharmaceutical widely:
Hypersil™ PREP HS C8 HPLC Columns by Thermo Scientific