Reversed-phase C-18 columns that have been equilibrated with acetonitrile and eluted

Reversed-phase C-18 columns that have been equilibrated with acetonitrile and eluted with gradients consisting of acetonitrile/water mixtures are very commonly used to separate mixtures of polypeptides. What do the terms “reversed phase” and “C-18” refer to?     https://www.studyhelpus.com/Questions & Answers/reversed-phase-c-18/