I am unsure if this link will help or hinder your understanding of the distinction I am attempting to draw, but, basically, jams retain pieces of fruit and jellies do not. Counterintuitively, my experience (which TLC corroborates) is that they tend to be softer than jellies despite that. An additional complication is that I have never detected any bits of grape in grape jams back home, but perhaps they had just been pulverized (grape jam is still far easier to spread than grape jelly, and thus preferred for sandwiches.) At a guess, I imagine the difference is that jelly makers add more pectin since they do not have the assistance of the actual fruit to make gelatin.

With regard to PB&J sandwiches, I prefer jam because jelly usually forces me to mash or slice it with the knife before it can be spread evenly, whereas I can usually smooth jams out with a single quick pass. Incidentally, if attempting to commit PB&J sandwich, you should consider whether you prefer the prospect of a little jam in the peanut butter jar or vice versa unless you just use a second knife (I find peanut butter in the jam/jelly jar eminently preferable to the reverse, because peanut butter does not require refrigeration. ) I do HIGHLY recommend it; if you have ever tried a peanut butter sandwich (for which I do not care, though many swear by it) you will find that the presence of gelatin makes PB&J much less sticky by comparison, greatly reducing the "roof of the mouth" factor.