It's tinfoil to spin wild theories as to just what the purpose of these things are, but there is a difference between the two. Contrails eventually dissipate and are gone. "Chemtrails" don't, and linger. On days that have a lot of them, they'll spread out and merge into a milky, hazy cloudcover. The sun can be seen through this layer of whatever it is, with a sort of sundog effect around it, like a refracted halo.
Again, I have no idea what the deal is with this, but there is a difference. If I were to speculate, my guess would be a misguided attempt at minimizing "global warming."
The trails are the same. The difference in disipation is a matter of atmospheric conditions. If the upper air is still, especially if it’s under a pressure system, the trail will maintain shape for a long time; if the upper air is mobile (has a breeze) they’ll disipate quickly. It’s just like any other cloud, the less it’s stirred the more it stays.