You can interchange players at any position, but from what I've found, this is the best explanation:
Point Guard (PG) is usually the facilitator of an offense. His primary duty is to call plays and get his players in the right position. His most important skills are Passing, Outside Defense, Handling, Jump Shot, and Jump Range.
Shooting Guard (SG) is usually focused more on being in the right place to make shots, rather than giving the ball to others. His umost important skills are Jump Shot, Jump Range, Outside Defense, and Handling.
Small Forward (SF) is a player that needs to be more versatile and able to play inside in a pinch as well as on the perimeter. You could say his Inside skills and outside skills are equally important, but it makes a difference what strategy you use (a 3-2 defense needs a SF with better outside defense, but a 2-3 needs better inside defense. Likewise, Jump Shot and Jump Range will be more urgent in a Run and Gun offense than in Look Inside, but Look Inside will demand a higher Inside Shot and Driving.
This is just my understanding and approach, but others may have more in-depth opinions and answers, be they similar or different.