I know that this question is asked a lot of time but I don't understand or I think, I got lost when I was reading Introduction to algorithms
They said, "It is not contradictory, however, to say that the worst-case running time of insertion sort is Omega(n^2), since there exists an input that causes the algorithm to take Omega(n^2) time.".
I read many articles that Omega is the best-case. As I understand correct me if I'm wrong, that the best-case is Omega notation and why they said, "since there exists an input that causes the algorithm to take Omega(n^2) time.", I don't understand why they called it worst-case and why I can call it Omega(n^2), isn't Big O for worst-case.
Also I don't understand they said, "Theta is notation is a stronger notion than Big O". Why is that? and When should I call an algorithm that it's Big O of whatever, Theta or Omega?
Because I'm confused and I don't know which one is for or how to use them.