What are the key differences between reverse phase and normal phase HPLC techniques?

Reverse phase and normal phase HPLC techniques differ primarily in the polarity of the stationary phase and mobile phase. In reverse phase HPLC, the stationary phase is nonpolar and the mobile phase is polar, while in normal phase HPLC, the stationary phase is polar and the mobile phase is nonpolar. This polarity difference affects the retention and separation of compounds in the sample.