Apricots, peaches and nectarines all belong to the Prunus family of fruit trees (which also includes plums), so they are very closely related and thus understandably easy to confuse with each other. Sometimes these fruits are collectively called "stone fruits" because they all share one characteristic in common -- a large, hard pit or stone in the center that acts as the fruit's seed.
The pit inside stone fruits closely resembles an almond, which happens to be another close relation. In fact, they can be used to infuse an almond flavor in liqueurs, custards and similar dishes.
Peaches can be spherical or flattened, with bright yellow flesh or with white flesh tinged with pink. Their skins are fuzzy and exhibit a range of shades from red to pink to yellow to white. When ripe, peaches have semi-soft, almost creamy flesh. They give slightly when you squeeze them. They are famously juicy and, at their best, sweet and floral. They can be eaten out of hand or used to bake a classic peach cobbler. Peaches also marry well with pork; try a pork tenderloin with peaches, balsamic vinegar and rosemary for a winning main dish.
Peaches come in two main varieties: freestone and cling. As their monikers imply, the pit in freestone peaches is loose and easy to remove, while the flesh in cling peaches sticks tightly to the pit and will need to be carefully cut off if used for cooking.
Nectarines are extremely similar to peaches both in outward characteristics and in their genetic profile. Their main difference is that nectarines are smooth-skinned while peaches are fuzzy. Nectarines can also have a slightly tarter taste and somewhat firmer flesh. They might be more widely available if it wasn't for the fact that they are a little fussy to grow successfully and often subject to diseases. Nectarines can be used in any recipe where peaches are used.
Because of their smooth skin, you do not have to peel nectarines before adding them to a cooked dish.
Apricots look a lot like small peaches; their skin is slightly less fuzzy but not as smooth as a nectarine, and their coloration both inside and out is similar to yellow peach varietals, although apricots are more likely to be a solid orange-yellow with just a touch of red-pink blush whereas the coloration of peach skins is more variegated. Apricot's flavor, however, is distinct. Apricots tend to have a noticeable tartness to them. This tart quality makes them particularly good for baking, adding a complexity to the finished dish. Apricots are also often found in dried form, where they can be used in anything from granola or an oatmeal topping to a Moroccan lamb tagine or as an accompaniment to sliced duck breast.