Cost of capital is the minimum rate of return that a company must earn on its investments to maintain its market value and attract funds. It is used to evaluate investment projects and determine the company's capital structure. Cost of capital can be calculated for specific sources like equity, debt, preference shares, and retained earnings using various methods. The overall cost of capital is the weighted average of the costs of each source based on their proportion in the company's capital structure. It is an important concept in financial management for capital budgeting, valuation, and performance evaluation.