When shopping for your Canon lenses, you will likely run into 2 versions of the same lens. One will be the "USA" model and the other will be denoted as "Gray/Grey Market", "Imported" or "Direct Import".
What is the difference?
The "USA" Canon lenses are imported to the US by Canon USA. "USA" Canon lens warranties are supported by any Canon service facility worldwide. They are also eligible for Canon lens rebates - both instant rebates and mail-in rebates.
The "Gray/Grey Market", "Imported" or "Direct Import" Canon lenses are directly imported by the retailer or their import agent. They are not covered by the manufacturer's warranty. At least they "say" they are not covered. Word on the street is that Canon covers gray market lenses under their standard warranty if an invoice from an authorized dealer can be produced. This mirrors my experience as well.
Most reputable retailers will provide their own same-as-manufacturer's warranty on "Gray/Grey Market", "Imported" or "Direct Import" Canon lenses. Instead of sending the lens to Canon, you send it to the retailer.
What physical difference is there between USA and Gray Market Canon Lenses?
Usually none - in fact, I have yet to find a difference. These Canon lenses are made in same factory using the same parts and are identical in the finished product form. Reputable retailers would make any differences clear before you bought the lens. Scam retailers will give you lines such as "You have ordered the plastic mount gray market version of this lens". If you hear that line or any similar upsell tactic, consider a red warning flag raised.
The attractive difference in a Gray Market Canon lens is price. The imported version is nearly always cheaper - sometimes even cheaper than with the rebates offered on the USA versions.
So which should you buy? I'll let that decision up to you, but will say that I have far more than one copy of the "Gray/Grey Market", "Imported" or "Direct Import" Canon lenses.