The point is for your class to specialize in a subset of its abilities, and get some new ones, that fit your playstyle. In the case of Jedi Shadow:
Kinetic Combat focuses on defense, damage mitigation and gettting "aggro" (focusing the enemies' attention on you), so it's better suited for tanking (playing in a group where you take most of the damage and keep the enemies from attacking the damage dealers and the healer).
Infiltration focuses on stealth. A Infiltration Shadow can do a very high amount of damage very quickly, as long as he's able to start the combat while cloaked.
Balance improves your incapacitating abilities and seems better suited for PvP.
Note that you don't have to put all your points in one tree branch. Most "builds" (sets of point assignment) usually are a combination of a main tree and a few points from the others.
This post in the Jedi Consular forums has better descriptions of the different skill trees and sample builds that you can use: http://www.swtor.com/community/showthread.php?t=378888