A quick tip o’ the day. I also posted this answer on WikiAnswers for the question, “What is the cardinality of a data type eg integer?“
“Cardinality is the total number of unique occurrences of an entity (e.g. person, organization, or transaction) that can be included in a relationship.
A URI/URL, e-mail, phone number should be unique, providing a high cardinality. This means that each of the entity values would be represented at least once in a relationship.
A lower cardinality value would be something like a bit, or boolean value. This value will only occur twice before becoming redundant in a relationship.”