General Information
The word nut comes from the Old English hnutu, from Germanic.

It is the general term used to refer to an edible kernel (fruit) encased in a hard or woody shell. It is generally preceded by the name of the tree, its country of origin, etc.:

cashew nut, walnut, Macadamia nut, Brazil nut, hazelnut, etc.

Also note in the same family:

almond, pistachio

Despite their name, peanuts are not true nuts. While nuts are technically fruits, peanuts are actually legumes.

Nuts - Benefits for the heart 

According to the Faculty of Medicine at the University of Loma Linda, California, nuts can actually reduce the risk of heart disease because they are high in fiber, mono-unsaturated fats and antioxidants (vitamin E and selenium). However, they're also high in calories, but a serving of 30-50 g per day could be considered reasonable.

Sprinkle some nuts on your salad; serve them with cheese and grapes as a snack.
Use walnut oil in your salad dressings or for stir-frying.