Ok, here's a jam and jelly side by side, from the same company:
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On a saucer, the jam is on top, the jelly on the bottom.
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As you can see, the jam is a bit lumpier, and the non-lumpy bit is thinner, it spreads out all on its own.
The jelly is completely smooth, and it's a good bit more firm than the jam, but no lumps of fruit at all, and it does not spread on its own. It is not, however, as firm as jello, nor is the texture the same as jello, nor is the taste the same.
Between this particular jam and jelly, the jelly tastes much sweeter to me, almost sickly sweet. That's why kids love it.