There is a very simple way to tell the difference between segmentation and clustering.
As in most data analysis, a picture tells more than a 1,000 words. So I asked google pictures.
For clustering, google gives us the following search result (first page of results)
For segmentation, google comes up with a totally different catch:
See the difference? Clustering (first picture) is all about points in a (multi-dimensional) space, how to calculate the clusters, hence a mathematical exercise. Segmentation (lower picture) is all about people: our customers, how we consider them to belong to different groups/clusters.
For a more elaborate discussion about this difference you should definitely read my most successful blog post “the difference between segmentation and clustering“.