1. A method of assigning a document to one of a plurality of clusters of documents containing related content, the document having at least one feature, each cluster being associated with a cluster summary describing the content of the documents in the cluster and comprising a summary of one or more features of the documents in the cluster, wherein the summary comprises an aggregated feature vector that describes the set of documents summarized, the method comprising: determining, at a document clustering system, whether the document should be grouped with one or more previously created cluster summaries by comparing the at least one feature of the document with each previously created cluster summary and evaluating the similarity therebetween, the previously created cluster summaries being stored in a memory in a B-tree data structure; determining that the document should not be grouped with the one or more previously created cluster summaries, and creating, at a document clustering system, a cluster summary based on the content of the document and storing the created cluster summary in the B-tree data structure, wherein the leaf nodes of the B-tree store the cluster summaries; identifying at least one outlier in the plurality of cluster summaries associated with each of the plurality of clusters; attempting to merge the at least one outlier into other cluster summaries; incrementing a merge attempt count each time a merge is attempted; and removing the outlier from the B-tree data structure when the merge attempt count exceeds a predetermined threshold.
7. The method of claim 1 , further comprising: cleaning up the B-tree by reducing the number of cluster summaries in the B-tree.