Provisional tax is not a separate tax from income tax. It is a method of paying the income tax liability in advance, to ensure that the taxpayer does not remain with a large tax debt on assessment. Provisional tax allows the tax liability to be spread over the relevant year of assessment. It requires the taxpayers to pay at least two amounts in advance, during the year of assessment, which are based on estimated taxable income. A third payment is optional after the end of the tax year, but before the issuing of the assessment by SARS. On assessment the provisional payments will be off-set against the liability for normal tax for the applicable year of assessment.

source www.sars.org.za