CPC ka confusion - Suits and Res Judicata ka kya relation hai?

Main samajh nahi paya ki CPC mein suit aur res judicata ka kya relation hai. Sab kuch clear dikh raha hai, fir bhi yeh dono concepts mere head mein confusion pahuncha raha hai. Suit kya hai? Ek suit ek civil case hai, lekin phir res judicata kya hai? Ek aisa rule jo do cases ko relate karta hai. Lekin kya yeh dono alag-alag cheez hai ya yeh ek hi cheez hai? Main suna hua hai ki res judicata ki waja se ek case ko court mein present nahi kiya ja sakta, agar woh case already decide ho chuka hai.