Even though it is green, the avocado is actually a fruit that comes from a flowering tree. It's got that big seed pit in the middle -- like with a peach or an apricot, that's the key to knowing it's a fruit. It is also sometimes known as an avocado pear or an alligator pear, due to its looks (although the tree was once known by a much more colorful name).
While it is indeed a fatty fruit, it is also very high in nutritional value. From the Wiki entry:
Though the fruit does have a markedly higher fat content than most other fruits, most of the fat in avocados is monounsaturated fat, which is considered healthy in the human diet. A whole medium avocado contains approximately 25% of the recommended daily amount of saturated fat. Avocados also have 60% more potassium than bananas. They are also rich in B vitamins, as well as vitamin E and K.
So it's the good kind of fat - the one we're supposed to get from nuts and olive oil and all those Mediterranean-type foods. It also gives you about 25% of the RDA of Vitamin C, like a good fruit should. From a Vegetarians in Paradise article:
While other fruits gain sugar as they ripen, the avocado's sugar content decreases as it matures. It contains more protein, potassium, magnesium, folic acid, thiamin, riboflavin, niacin, biotin, pantothenic acid, vitamin E, and vitamin K per ounce than any other fruit.
It's also supposedly an aphrodisiac..... ;-)
Since a 1 cup-serving has 240 calories, 22 grams of fat, and 10 grams of dietary fiber, that calculates out to be 6 points. And 1 cup is a lot of avocado - you're not likely to get that much in just an order of California rolls or diced up on a salad. As long as you're not inhaling guacamole, you'll probably be eating less than those 6 points most times that you have it as part of a balanced meal.
So my verdict.... is that in moderation, the avocado is definitely a friend. Enjoy!