What is the difference between reversed phase and normal phase in column chromatography analysis? my sample is unknown which mobile phase can be use?

In normal phase chromatography, you use polar stationary phase such as silica and start elution with less polar mobile phase such as hexane and then start polarity of mobile phase gradually. fractions obtained from column in start contain less polar compounds.

In reverse phase chromatography stationary phase surface is made non polar by addition of alkyl groups. mobile phase are more polar and then polarity is decreased. more polar compounds elute first in this techniques. alkaloids, flavonoid glucosides can be separated by using this technique. Stationary phase is reusable and mobile phase solvents are less toxic to environment. 

for unknown compound you should use TLC for selection of mobile phase