Kalk is Calcium hydroxide mixed with fresh water. It is dripped in at a rate to replace evaporation, and keeps calcium, alkalinity and ph hopefully appropriate levels.
A calcium reactor is a more complex and expensive system that receives water from the system, lowers the ph of the water through carbon dioxide injection, and passes the water over calcium carbonate, which then dissolves a bit and the effluent is returned to the system.
A beginner should start with kalkwasser, in my opinion.