How do different string gauges affect my playing?

From Fretlight Knowledgebase

Currently Optek sells 2 gauges of string: 9's (light) and 10's (medium). 9's are generally for faster/lighter playing. If you are a beginner, we would recommend you use 10's to start out with. They last a bit longer (being a thicker string) and will help build up calluses on your fingers. People who do a lot of bending and pulling need thicker strings. Remember that either of these two string gauges will exhort a different amount of tension on your guitar neck, and the neck WILL need adjusted to compensate. Thus if you have had your guitar set up to play great with 9's, if you put on a set up 10's (which would cause more pulling tension on the neck) you would need to adjust the neck back a bit with the truss rod.