Categorical (also called nominal) data can take on a limited number of possible values called categories. Categorical values "label", they do not "measure". Please use [ordinal-data] tag for discrete but ordered data types.
For analysis, categorical values are considered as abstract entities without any mathematical structure such as an order or a topology, regardless of how they are coded and stored. For more, see http://en.wikipedia.org/wiki/Categorical_variable.