Difference between Medoid and Centroid

Question

For a certain dataset, I am finding the k closest points in my dataset to the centroid of the dataset. Are these k points the k medoids or are medoids something completely different. If so, is there any mathematical term for the points that I'm finding? I use 2 distance metrics, one is the Mahalanobis distance and the other is a weighted average of Euclidean and cosine distances.