An Ore is an economic entity, not a physical one. It is defined by the presence of an economically valuable concentration of some material.
However, times and markets change. Today's waste may be tomorrow's gold mine (or perhaps silver mine - exactly what happened to the waste from one region's gold mines). Ores too dilute to be mined may become valuable as technology changes. For example, there are huge copper mines which recover copper from ore of only 3% copper, not feasible a hundred years ago.
Note that some ores are mined primarily for one material (such as lead), and another (such as silver) is recovered as a bonus. In this way, the mineral Galena (Lead Sulfide) is the leading ore for silver.
ORES are typically not isolated - that is, many different ores are often co-located within a regional "ore body". Ore bodies are formed through a combination of geology (such as ocean bottoms, volcanoes, or subducting tectonic plates) and chemistry (due to a natural concentration of some collection of elements, plus a chemical environment such as an oxidation zone). Many different ores - for many different metals - are thus found in regional proximity.
One such example is a large body of magma deep within the Earth. As it slowly cools over thousands of years, minerals begin to crystallize as the temperature falls below their melting points. If these crystals have a higher or lower density than the magma containing them, they will settle down or float up, resulting in layers concentrating those minerals. Also, the "left over" minerals will be concentrated in the last sections to solidify.
Another example is a drying lake or sea. Precipitation (rain) dissolves minerals over a wide region (usually the most soluble salts first), and rivers transport these chemicals into a body of water. But when conditions are dry, the water will evaporate faster than it can be replenished, resulting in concentrations of salts, carbonates, and other chemicals. Here, too, the layers tend to isolate specific minerals, as the chemicals with lower solubility tend to precipitate first, and those with the greatest solubility (such as halite) are last (and thus on top).
A somewhat similar circumstance often occurs where an intrusion of magma provides a heat source for groundwater. Under the high pressures and temperatures deep within the Earth, water may dissolve many compounds we normally think of as insoluble, such as quartz. As the water flows out to the surface, it very gradually cools, the pressure falls, and chemical compounds precipitate (or crystallize) at different points, resulting in the concentration of different compounds along the water's path. The quartz deposits in Hot Springs, Arkansas are a good example.
A related process is called supergene enrichment. Supergene enrichment occurs when certain metals are leached out of slightly soluble minerals by hydrothermal fluids. These metals are then redeposited as a different mineral in concentrated pockets. These pockets are usually more rich in ore metals than the original material, thus the process is aptly termed an enrichment. Chlorargyrite is an example of a supergene enrichment mineral.
At some point, an ore body is brought to the surface, where erosion exposes it, and water and oxygen potentially induce still more changes to the chemistry. Plate tectonics can also plunge these rocks deep within the Earth, where intense heat and pressure metamorphize the rocks, often inducing the formation of additional minerals. These cycles can repeat many times, resulting in an evolution of mineral species.
An ore for one metal (such as copper) may consist of many different minerals, while an ore for another metal may consist of only (or at least primarily) a single mineral, in some cases the pure metal (such as gold).