It is a type eof learning in which , it has unlabelled data (i.e you have input data but donot have output data).

example: you give a collection of fruits of image and say system to  identify  all fruits in image then , machine used allgo and learn features of different fruits in image (like color,shape,size) and those fruits which are similar are divided and put in one group called as cluster.

Unsupervised learning is of two type to solve the eproblems

1.Clustering (k-mean clustering,hierarchical clustering)

2.Association (Apriori)