Most data are available in case form with data for each sampling unit. Sometimes data are not available for each unit but are already summarized by counting the frequency of occurrences of each value, called frequency form data.
Whenever possible you should record data in case form. Case form data can be reduced to frequency form, but frequency form data cannot be reconstructed into case form. You may want to use frequency form data when working with a large amount of data from a database, where you can save computing resources by letting the database server tabulate the data.
Each column is a variable (Height, Eye color), each row is a separate case (Subject) with the values of the variables on that case.
Each column is a variable (Eye color) and a separate column for the number of cases (Frequency), each row is a combination of categories with the frequency count.