As a follow-up to yesterday's post and Matthew's comment on it:
In the purest scriptural sense, I view "transgression" as everything that keeps us from becoming like God. I believe many of those things are the natural result of "the Fall" - or, in other words, are the by-product of Adam's transgression. I believe our 2nd Article of Faith teaches that we have been redeemed / saved from many of those things already, since we didn't choose them willfully.
I view "sin", on the other hand, as those things we intentionally choose to do (or not do) that keep us from becoming like God - knowing or believing that we need to do (or not do) them to become like God. I view "repentance" as the process of trying to change our nature and become more Godlike - so, at the most basic level, I view sin as NOT trying to repent (change). To me, it's really not more complicated than that.
I believe sin generally is defined communally by those who want to tell others what to do and not do for the overall good of the collective group (and I have no problem with that general concept, even as I believe much of what is considered to be collective sin is somewhat arbitrary and not "eternal law"), but I believe sin is defined by God on an individually sliding scale - since He is the only one who can look upon the neurology, heredity, environment, societal norms, intelligence, spiritual sensitivity, etc. of each child and know exactly what that child is capable of doing - and what actions are willful rebellion vs. ignorant transgression - proactive vs. reactive - simple correction vs. deeply ingrained inclination - honest mistake vs. intentional error - etc.
In other words, I believe that what is "sin" for me might not be "sin" for another individual (although it all is transgression) - and that there are VERY few things that are "sin" for every single individual in every single situation (rather than sin for one and transgression for another or sin in one situation vs. command in another). Perhaps the best example of this in all of our recorded scriptures is Nephi and Laban.
A modern example is also instructive:
Did each and every person who participated in the attacks on 9/11/01 commit sin in doing so - or were the actions of one or more accounted to them as transgression, given their upbringing and youthful indoctrination? I don't know - and I don't take time or spend effort trying to decide.
I believe that "Judge not, that ye be not judged" is MUCH more universal than many people think - and that our 2nd Article of Faith is MUCH more powerful than even most members realize.