International editions of textbooks are meant to be sold outside of the United States. In the same way that many different products are sold with wildly different price tags in different countries (think medicine), textbooks have drastically different prices in different counties. Those books sold outside of the United States are "International" textbooks (at least to all of us in the States!). Because of these differences, it is often possible to save a ton of cash on international editions.
The difference between these two versions is always superficial. Sometimes the cover will be different or have different markings on it, but many times there will be no differences at all! Either way, you can be assured that the book itself will have the exact same content/chapters/pages as the standard US books.
Yes, buying international edition textbooks is perfectly legal. Despite the books industries best efforts to make it seem like it isn't. The key is that it isn't legal to resell international edition textbooks any place except for the intended country of sale, but anyone, from any country, can buy them for personal use. If you want to read more, you can check out this article on Time.com:
EVERY book has an ISBN (International Standard Book Number).
It's a 10 or 13-digit code, usually on the back of the book, near the bar code.