An ore is a mineral deposit that can be mined - generally it has a high concentration of metallic minerals. Many minerals (ie quarts, feldspar) have no extractable metal. Ore minerals are often sulphides (ZnS - sphalerite, PbS - galena). Often gold is mixed with quartz.
Metallic minerals (ie ore) are generally found in veins formed by the intrusion of hot hydrothermal fluids into rock fractures.