Ethylene is the simplest of the plant hormones, just a pair of carbons and four hydrogens (technically it should be ethene according to the current system of nomenclature, but the old name has stuck). In basic terms, it serves to prod plant development forward, hastening the opening of flowers, the shedding of leaves, and, importantly, the ripening of fruit. This is the reason behind the old trick to ripening tomatoes by putting them in a bag on the counter (to concentrate the naturally produced ethylene) often with an apple (to produce even more ethylene.) It's produced as a reaction to stress, which is why the ancient Egyptians were able to speed ripening in sycamore figs (Ficus sycomorus) by making cuts in them.
Ripening fruit is a good thing if you're ready to eat it, but if you're trying to store the fruit, especially for an obscenely long period of time, you want it in as close to stasis as you can get, not pushed along by ethylene. Enter 1-methylcyclopropene, aka 1-MCP, aka SmartFresh. 1-MCP has some chemical similarities to ethylene, but it turns out to mostly inhibit the effects of ethylene by preferential attachment to the ethylene receptor, allowing fruits to be stored for otherwise impossible lengths of times. The results aren't quite the same as fresh-picked, but in some cases it may be close enough. Although there are questions whether the ability to sell year-old apples is really a good thing.
Anyhow, this was basically intended as a lead in for David Karp's new article on 1-MCP (and apples, mainly) in tomorrow's New York Times, which through some miracle of time travel, you one can read already, here. (Free log-in required...eventually the story won't be available for free, but by then it ought to be copied illegally a half-dozen places if precedent holds.)
Update: For a look at the effects of 1-MCP, take a look here.