According to Wikipedia at http://en.wikipedia.org/wiki/Grey_market
A grey market or gray market also known as parallel market is the trade of a commodity through distribution channels which, while legal, are unofficial, unauthorized, or unintended by the original manufacturer. The term gray economy, however, refers to workers being paid under the table, without paying income taxes or contributing to such public services as Social Security and Medicare. It is sometimes referred to as the underground economy or "hidden economy."
According to B&H Photo at http://www.bhphotovideo.com/find/HelpCenter/USGrey.jsp
The terms "grey market," "direct import," and "IMP," refer to the same thing in this article.
B&H sells most products with a warranty from the manufacturer or from the manufacturer's licensed importer. We also sell some products we've obtained from sources other than the manufacturer or its licensed importer. These are "grey market" products.
"Grey market" is not illegal, not factory seconds, not demo merchandise, not cheaper or inferior products. In fact in almost every instance a "grey market" products is absolutely identical to its US-warranted counterpart. "Grey market" and US-warranted products are manufactured in the same factories from the same components, and sub-assemblies, to the same specs and tolerances, by the same workers. In terms of the item itself (excepting PAL video -- see below) there is no difference at all. A "grey market" Nikon 50mm f/1.4 D-AF lens (for example) is exactly the same in every possible way as the US-warranted version.
Gray market means that this lens is not distributed officially by Canon in the USA, so you don't have warranty of Canon USA, that's why is cheaper.
The lens is the same lens, so if you don't care about the warranty go for it.
It schould work on all Canon Rebel and on Canon 350 up to Canonn 700. Note that in Europe the types are CanonD 350 and so on up to CanonD700. Exactly the same cameras are called Canon Rebel T1 and so on up to T4 and more in USA. This is confusing but Amazon should know.
No. it is a telePHOTO lens with auto focus and image stabilization features. It zooms from a 55mm to a 250mm focal length. if you added a 2x teleCONVERTER to it, it would then function as a 110 mm to 500 mm lens. The main trade offs if you do this are
1. requires much more light from the same subject.
2. The less expensive telephoto lens do not necessarily allow the image stabilization and auto focus features to work.