Market share refers to that portion of the total market that is controlled by a particular entity. E.g. If Acme Corporation manufactures a product X in the Chemicals market. There are ten other companies making ten other products. The total sales of all these products put together are $ 100 million. The sales of X are $ 10 million, then it is said that Acme Corporation has 10 percent market share.
But real world business is not as simple as this; market share could also refer to the sales in a particular geographic area. It could also refer to a particular distribution channel. E.g. market share in the original equipment manufacture channel. In most markets companies will have more than one product. In such cases market share will be denoted in terms of target segments. E.g. high-end segment, middle segment, lower end segment etc. Companies are constantly thinking of ways to increase their share in the market.
But real world business is not as simple as this; market share could also refer to the sales in a particular geographic area. It could also refer to a particular distribution channel. E.g. market share in the original equipment manufacture channel. In most markets companies will have more than one product. In such cases market share will be denoted in terms of target segments. E.g. high-end segment, middle segment, lower end segment etc. Companies are constantly thinking of ways to increase their share in the market.