"Greenwashing" is the practice whereby a moderately green or even completely un-green product is given some variation of the name "eco" and marketed as the one and only choice you need to make to live lighter on the planet, you hippie. Like whitewash on a fence, something greenwashed might look green from far away, but flakes of green veneer start to peel away upon closer analysis.
Building Green makes a good list of the many types of greenwashing: which can take on the form or outright lying, specious arguments, claims that are simply poorly researched, making compliance with environmental law sound like something totally awesome and groundbreaking (no ozone-depleting chemicals!), highlighting one green aspect while ignoring a fatal flaw in the sustainability of the entire product, etc.
Greenwashing is a problem because it misleads customers and that's wrong in itself, but also because it undermines the real green efforts of truly green products. At best it contributes useless noise to a serious conversation, and at worst it allows companies and consumers to continue to dodge responsibilities for environmental externalities. It can also quickly become a bandwagon, but a bandwagon based on marketing and hype in the place of actual substance.
My favorite example of greenwashing is a product I ran into this spring in my friend's lunch box on a canoeing trip: EcoZip Plastic Bags.
If you go to the website, do take the time to watch the short flash video touting the truly green power of the EcoZip: which is that by using a plastic bag with TWO sides, you can pack TWO lunch items in one zip-closing plastic bag, thereby throwing away less plastic and truly being "ECO."
While sure, that's not a bad idea, it probably saves a little plastic, and the plastic bags ARE made without the hormone-disrupting BPA and that deserves recognition sure, still it's a stretch to call such a change "Eco" when you could just pack your sandwich in reusable tupperware. Or just put two things in a plastic bag anyway, for that matter. This is trying to get the market share of the people who just go for the Eco item without thinking, and that is absolutely all this product accomplishes. The company is in the industry to make plastic bags, it is not in their interest to have you actually use less plastic.
You've got to be careful with greenwashing critique of course, because there are always ways that things could be improved as well as various ideas about what sustainability means even among people who are dedicated to it. Pretty much everybody engages in small acts of greenwashing when they use statements like "help protect the environment, (buy this)," because unless they are actually out there buying up land to put into conservation with every product they sell they are probably just providing a solution that does less active damage than an alternative--and that includes pretty much every aspect of my industry of green building. It would be completely stupid to let the goal of what is perfect get in the way of what is merely better--but at the same time, come on, we can do better than a two-sided disposable plastic bag.