Clustering is an important tool to search for unknown structure in data.
Its aim is to find "clusters" hidden in the data, i.e., to identify groups of data points that belong together. For clustering, no prior knowledge is necessary -- we look for structure that is intrinsicly present in the data. Thus, cluster analysis belongs to the field of unsupervised learning. Nowadays applications come with big and high dimensional data which makes automated cluster analysis a key step for evaluating data.