What is the difference between conventional milling and cline milling?

You may be looking for the difference between Climb vs Conventional milling. Also known as Down milling and Up milling respectively.

In short, the difference is the direction that the teeth of the cutting tool move relative to the material. This can primarily affect smoothness of resulting cut, tool life, and the force on the machine and material. Better description can be found in these links:

http://en.wikipedia.org/wiki/Milling_cutter#Conventional_milling_versus_climb_milling

http://clinetoolmexico.com/pdf/fresado_tecnica.pdf