If you're Googling for information on clusters (such as you might find here), then don't do as I did and let your right index finger slip from the "U" key over to the "Y" key and then click the first link that comes up ... because it may look a little like a medical information page at first ... but it isn't ...

Of course, given the topics under consideration here, the name of this book's author is ... strangely appopriate ...