Photo taken from Google, all credits to owner.
Common practice and most law of the US ( varies state by state) is that the LEFT lane is for passing or faster moving traffic. Now, depending on the situation, you should stick to that rule and if you don't at least have the common courtesy to let people in who need to get into the left lane! Hogging the road, driving slow in the left lane, and or not letting people in when you can is just plain annoying and quite frankly foolish. You don't own the road, so drive friendly ( Texas motto) !