Olives are preserved in a brine consisting of a strong solution of water and salt to preserve them - that is why they seem so salty to you. Because olives are so bitter when picked, they need to be cured for consumption. Many of the olives that are grown are picked green (unripe and they are very UNEATABLE until cured) and then they turn black after the curing process (lye curing - usually wood ash - and oxidation). Then they are usually packed in a weaker solution for sale. Olives can also be brined in a salt solution (no lye) as well as dry-cured - by rubbing salt on them.
The darker the olive - the higher the oil content - meaning a richer flavor. Nicoise and Greek Kalamata are best when picked at full maturity.
Because of the strong flavor and saltiness of olives you need to be careful when cooking with them as they can dominate the dish you are preparing.
Kalamata olives are brine-cured and jarred in vinegar. They are usually meaty with a fruity taste. Marinated in oil gives them a smokier taste. Good for salads.
Manzanilla are small green olives usually pitted and stuffed with pimientos. Used in martinis or certain dishes.
Nicoise are tiny/ brown and brined-cured and usually packed with herbs. Good for salads.
Black olives are soft skinned and vinegar-cured. Good for salads.
Various countries prepare olives in different ways. Marinated in oils and/or vinegars or citrus, stuffed with a variety of things, packed with herbs and spices, etc.
Olives (depending on the type) can be used in anything from drinks (martinis), breads/biscuits/bagels/muffins, salads, veggie dishes, meat dishes, on pizza/bruschetta/crostini, used in tampenades, eaten right out of hand, etc.
Personally - I love them in salads (greens or pasta or rice) and I really enjoy them as part of a cheese/olive lunch or snack. I was brought up on the ?Mediterranean way? of eating them - cheese and olives can make a meal with a chunk of homemade bread. When I get the munchies I grab for them over junk foods.....a little dish of olives, pickles and pepperoncini and/or banana peppers - satisfies my craving for junk.