Market Share is the percentage of the entire market accounted for by a company/brand/offering. The share of the market can be defined in terms of dollar share or unit share.
For example, if a company X selling toothbrush has sales of Rs. 200,000 and if the entire market for toothbrush is Rs. 500,000, then the market share of X is 40%.
For example, if a company Y selling two wheelers has sold 500 two wheelers and the total number of two wheelers sold is 4000 then the market share of Y is 12.5%.