Provisional tax is not a separate tax. It is a way of paying income tax during the year on income that nobody withheld PAYE from, so you are not hit with the whole bill at assessment.
You are generally a provisional taxpayer if you earn income other than remuneration that has already had PAYE deducted. Freelance and consulting fees, rental income, business profits as a sole proprietor, and significant investment income all qualify. Companies are provisional taxpayers automatically. Someone earning only a salary with PAYE deducted normally is not, and pensioners over 65 are excluded in certain circumstances where their income is limited to interest, dividends and rental below set thresholds.
There are two payments. The first is due by the end of August, halfway through the tax year, and the second by the last day of February. A third, voluntary top-up can be made by the end of September to limit interest if your estimates were low.
The part that costs people money is under-estimation. If your second-period estimate falls too far short of your actual taxable income, SARS levies an under-estimation penalty on top of interest, so a deliberately conservative estimate can be an expensive way to manage cash flow.