An object contains a 150 by 4 data matrix and a 150 by 1 cluster membership vector.
There are 3 balanced clusters in a 4 dimensional space spanned by the 4 columns.
References
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
The New S Language.
Wadsworth & Brooks/Cole. (has 'iris3' as 'iris'.)