1. A method, executed by a computer system, of forming a unified classifier for a category, the method comprising: storing a taxonomy of categories, the categories for labeling videos; accessing a video repository; retrieving from the video repository a training set of videos for a category from the taxonomy of categories, the training set comprising a positive training set of videos labeled with the category, and a negative training set of videos not labeled with the category; forming a plurality of positive bags, each positive bag comprising a plurality of the videos from the positive training set; forming a plurality of negative bags, each negative bag comprising a plurality of the videos from the negative training set; applying a boosting algorithm to obtain a plurality of sets of weights, wherein each set of weights comprises one weight for each bag, the weight for each bag based on a weight of each video in the plurality of videos in the bag; training a unified classifier, wherein training the unified classifier is based at least in part on the plurality of positive bags, the plurality of negative bags, and the plurality of sets of weights; and storing the unified classifier.