The Date data type consists of a sequence that represents a date with day, month and year with numbers and separators (ex.: 01/01/2013 or 2013/01/01).
In many places like when adding a date column to an object or creating filters it's possible to select a date part (YEAR, MONTH, DAY, TRIMESTER, ...) or combinations between them to be used.
The data type of a column is defined in the data source creation, see Input Data Format.