Set cover problem NP completeness

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I have a quick question about Set cover problem. In Wikipedia, < https://en.wikipedia.org/wiki/Set_cover_problem >, they are saying "The decision version of set covering is NP-complete, and the optimization/search version of set cover is NP-hard". What is the decision version and what is the optimization version? I am really confused here. I used to learn that set cover is an NP-hard problem. Many thanks!