How to decode tread patterns?
Ask the guys you ride with and also complete strangers you meet on the trail, what they use and why. My guess is 9/10 times it was on the bike already, or it was on special. 

I have ridden for years, have a range of tires I have tried. Some I have liked better than others, mostly its a trade between speed, traction for braking and acceleration (not my strongest point anyway) and cornering. 

A tire is a compromise of the following 


weight
cost 
Straight line performance (one and/or both directions)
Cornering (Predictability and maximums)
Puncture resistance 


Add to this the types of terrain being ridden (the list gets bigger)


Rock 
Dirt 
Sand (soft)
Hard pack
Pavement


Then add 


Wet
Dry


If you are a casual weekend warrior like me, you got to the LBS, ask them for a tire for the trails you regularly ride (which they should know) and hope they gave you something half decent.