I have done Lindy Hop and jitterbug workshops, and I could probably recognise the difference between them, but I'm finding it very hard to explain. There are far more similarities than there are differences.
If I remember right, the Lindy Hop was the earlier dance, and was a black street dance, evolving in Harlem. The jitterbug evolved from that, and became popular with white teenagers and GIs etc, and was therefore cleaned up a bit.
I know that the jitterbug hold is more upright and a bit further apart than the Lindy hold, which is very close, and has more bent knees and a more bent overy posture, needed for the acrobatics which the jitterbug doesn't have as much of.
Edit: I've just double checked, and rhythm is also a key thing - Lindy is danced to a fast 8 beat rhythm, whereas jive and jitterbug is usually on a 6 beat, apparently. Another difference is that the Lindy is usually slotted (straight in and out moves) whereas the jive and the jitterbug are more usually circular (dancers can move out in any direction)