What is Cluster analysis?

Cluster analysis is a technique whose purpose is to divide into groups (clusters) a collection of objects in such a way that:

1. The objects of the same group are the most similar possible.

2. The objects of the same group are the most similar possible (internal cohesion of the group). And the objects of different groups are as different as possible.

Source: http://scikit-learn.org/stable/auto_examples/cluster/plot_cluster_iris.html