You will find different types of salt on the market but select the best salt for your water softener. You should select the appropriate one from different forms (pellets, crystals, tablets, etc) and compositions (Sodium chloride, potassium chloride, or additives). Salt type selection also depends on the quality of water and daily water usage.
For example, High iron contaminated water needs special salt (with additives) that can clean iron properly from resin beads. For high water usage you can buy pellet form of sodium chloride. Crystal form is the perfect salt for small families and low water usage.