The short answer is no, there is no real difference between coconut water and coconut juice. Both refer to the clear, slightly sweet and refreshing liquid found inside young green coconuts. (Sometimes it also contains pulp or jelly.) Although coconut water/juice has long been enjoyed in tropical regions, it has only recently become all the rage in the US. Unlike coconut milk, the water/juice is fat free and low in calories. It's also full of electrolytes, such as potassium, and low in sodium, leading it to become known as "nature's sports drink." Besides drinking it straight up, coconut water/juice may be used in a variety of recipes.
We generally consider the juice straight from a young coconut to be the tastiest, while other brands or packaging are just a matter of preference. (Stephanie recently gave a positive review to Real Coconut Water from Taste Nirvana.) Some people find the canned varieties to be too metallic tasting. Another thing to look out for is added sweeteners, preservatives, or flavorings. Also keep in mind that brands targeted at the US market tend to be more expensive than those found in Southeast Asian and Caribbean markets.
To learn more about the health properties of coconut water and how it is farmed, check out this article from Mother Jones:
• Is Coconut Water Really Better Than Sports Drinks?