Two different questions

The first decision, and it determines everything after it.

	Directed	Undirected
Is there a target?	Yes — something specific we are trying to predict	No — we are looking for structure that is already there
The question sounds like	Which customers will leave in the next hundred days?	What natural groups exist among our customers?
Typical methods	Decision trees, logistic and multiple regression	Cluster analysis, association rules
Who decides if it worked	The data. Test it against cases held back	You do. Are the groups meaningful to the business?

That last row is the one that matters. A clustering will always produce clusters. Whether they correspond to anything real is a judgment, and only somebody who understands the business can make it.