In the United States, the word entree is often used to signify the part of a meal that you would think of as the main course.
In other parts of the world, however, it is not uncommon to hear the word entree used to indicate an appetizer, first course or starter.
For the most part, this confusion isn't too serious, unless you happen to be ordering in a restaurant in a part of the world where the word entree means the opposite of what you think it means. In which case, you might end up getting your starter as your main course or vice versa.
Take the following sentence, for example:
"For my entree, I'll have the soup."
In the U.S., saying that sentence will get you the soup as your main course. In most of the rest of the world, you'll get the soup as your appetizer.
If you're not sure, you could always specify "main course" or "appetizer," instead of entree, just to be clear.
See Also: À la Carte