I am a beginner in GIS and Learning PostGIS. I came across this two terms Geom and Geog, Could anyone explain in brief. What is the best to use with map data and Do all functions support both Geom and geog.
You can find your answer in "PostGIS in Action", from Regina O. Obe and Leo S. Hsu, Edited by Manning
The difference from geometry and geographyc type:
What is best to use? It depends:
Also, take a look in an already answered question here.
First, I would suggest reading this excellent answer (ahem!) on the differences between CRS, SRS, and projections.