The fundamental difference lies in their basis of calculation: normal tax liability is computed based on the company's taxable income as per the regular provisions of the Income Tax Act, after availing all eligible deductions, exemptions, and incentives. In contrast, Minimum Alternate Tax (MAT) liability is calculated on the company's 'book profit', which is the net profit as per its financial statements (prepared under Schedule III of the Companies Act, 2013), adjusted as per Section 115JB of the Income Tax Act.
Ultimately, a company is required to pay the higher of the two: its normal tax liability or its MAT liability. This ensures that even companies with significant book profits, but low taxable income due to various tax benefits, pay a minimum amount of tax. If the normal tax liability is lower than the MAT liability, the company pays MAT, and the excess tax paid (MAT over normal tax) becomes eligible for MAT credit under Section 115JAA.