It is a form of decorative concrete which is stamped with a pattern, before it has set, to make it look like other forms of flooring. Block paving for instance is very popular as is crazy paving.
too what area of concrete? the deck, the pool foundation? from my experience you don't' need to add it to any. but I'm curious on what others are doing. the only time you need to worry about salt water is if it is going to come in contact with metal objects. salt water pools should have a metal (i forget what kind of metal) disc that you can place in your skimmer basket. this type of metal errodes faster than the stainless steel of your ladders, etc. if your pool is an onground pool, salt water is not recommended due to the fact of evaporation and salt water coming incontact with steel walls. as far as i know concrete and salt water is not an issue, unless maybe is is stampcrete, in which case the sealent may be effected. the mineral contect of the water with salt is still very low, now where near ocean water. just put an extra couple layers on the stapecreate and you should be fine.