Jelly is Jam in UK speak, right?
That said, you can also make a PB&J with jam.
I would call US Jelly "fruit flavoured", with a high water content, no fruit solids, and occasionally even artificial fruit flavours; whereas jam is typically around 2/3 real fruit (before boiling), 1/3 sugar, with no added water, so has a denser and more "lumpy" texture.
Jelly also often comes in flavours not associated with Jam, like grape for example. (See also KoolAid, which is the US equivalent of Squash, but powdered, and also "watery" when made to manufactures instructions).
Generally no artificial flavors or colors are used (I checked a cheap jar of grape jelly in my fridge before writing this). Also because it's made like jam, it uses pectin as its thickener. The reason grape jam doesn't exist across the pond is because it's made with Concord grapes, which were bred in the US and have large seeds that essentially require straining - hence jelly.
UK jelly (US Jello), on the other hand, is basically water with artificial flavor/color, sugar, and gelatin added and allowed to set. Quite different IMO.