Sales tax is added to the price. 50% means to add 50 cents for every dollar you pay. 100% is double price.
50% tax on income, for instance, is taking half the money from your income.
Income tax is calculated differently than sales tax (but most of the time it's progressive taxation which means you only pay the the 50% rate on your marginal income and not on the whole: my marginal income tax rate is 41% but I pay only 9% of my income as income tax for instance)