1. A natural language understanding system using at least one hardware implemented computer processor for automatic unsupervised clustering of dialog data from a natural language dialog application, the arrangement comprising: a log parser configured to extract structured dialog data from application logs, the structured dialog data including a transcription of a dialog between a user and the natural language dialog application, the transcription being generated by the natural language dialog application; a dialog generalizing module configured to automatically generalize the extracted dialog data using different independent generalization methods to produce generalization identifier vectors aggregating the results of the generalization methods used, the generalization identifier vectors indicating descriptive categories of the different independent generalization methods that correspond to statements by the user and statements by the natural language dialog application included in the extracted dialog data; and a data clustering module configured to automatically cluster the dialog data based on the generalization identifier vectors using an unsupervised density-based clustering algorithm without a predefined number of clusters and without a predefined distance threshold.