If something is important, things can still happen without it. If something is essential, it is necessary. Nothing can happen without it. Bob is an important member of this team= Bob is valuable, but the team can still accomplish its goals without him. Bob is an essential member of this team= If bob is not here, the team cannot do its work. Lemons are an important ingredient in this cake recipe= if you don't add lemons, the cake will taste bad flour is an essential ingredient in this cake recipe= if you don't add flour, the thing you make won't be a cake at all.