In the Jewish Religion is there a specific name for this?

Its when the Rabbi says something and then the congregation says something after him. Its like their completing the sentence. Like if I said row row row your boat then you would say Gently down the stream. (not making fun just giving an example). Is there a specific name for this and what is its purpose. Why do they do it? I'm not Jewish I just attended a service recently for my college cl and now I need to know so I can complete my ignment! Thanks.