Normal phase vs reverse phase HPLC
Normal phase

Normal phase HPLC systems are similar to the flash-column chromatography that you might be familiar with. 

A silica stationary phase is eluted with a non-polar solvent such as hexane, or a fairly non-polar solvent mixture such as 2-propanol in hexanes. In normal phase chromatography, only organic solvents are used. 

In the normal phase, polar molecules elute slowly, and non-polar (greasy) molecules elute quickly. 



Reverse phase

Reverse phase is essentially the opposite of normal-phase. 

A non polar stationary phase (often silica in which the free hydroxyl groups are end-capped with something greasy, C18 chains are common but many many variants are possible) is eluted with a polar solvent such as acetonitrile/methanol, or a fairly polar solvent mixture (acetonitrile water mixtures are common, or methanol water mixtures). 

In the reverse phase, polar molecules elute quickly, and non-polar (greasy) molecules elute slowly.