There are two reasons you may have salt water in your pool. Many people have a preference to swimming in salt water and this can be achieved by merely adding up to 10 (25kg ea) bags of salt in a 50,000 pool. The most common reason however is for use with a salt water chlorinator.
Salt is actually Sodium Chloride. Salt Chlorinators work by breaking up the salt molecules and releasing the chlorine into the water. This is how they chlorinate your pool for you. They do require some maintenance but they save the day to day manual dosing.
There is no such thing as too much salt in a pool. While tap water has a very low salt level [0.03-0.15%], many ocean pools, such as those operated by local councils, are successfully treated and chlorinated with levels up to 3.5% or 35 ppt. A salt chlorinator by comparison usually operates on only 0.4-0.7% (4-7ppt) salt.
It is impossible to guess an accurate salt level and while some chlorinators test for salt they can only approximate. The best way to test your salt is to take a water sample to any poolshop, or you can even buy test strips now!! We recommend all salt chlorinators operate at between 0.6%-0.7% salt. This will also help extend the cell life.
Salt levels can be increased by the use of liquid chlorine which is sodium hypochlorite, i.e. contains salt, however it is only a small amount, unless used for extended periods.
This PDF factsheet contains seasonal maintenance information for owners of salt chlorinators on concrete pools.