[R] k-means clustering

[darteta001 at ikasle.ehu.es]

Dear list, first apologies for this is not strictly an R question but 
a theoretical one. 

I have read that use of k-means clustering assumes sphericity of data 
distribution. Can anyone explain me what this means? My statistical 
background is too poor. Is it another kind of distribution, like 
gaussian or binomial? What does it happen if the distribution is not 
spherical? Could you give me an example or a link to information about 
this?

Thanks for your help

David