You assign to each column a datatype, which defines the data values allowed for that column. For example, if you want to include only names in a column, you would assign to that column a character datatype. Or, if you want a column to contain only telephone numbers, you would assign to the column a numeric datatype.
To define a column's datatype, you can use the SQL Server - supplied datatypes, or you can create your own datatypes when the system datatypes do not meet your needs. For details on system datatypes, see the Datatypes topic in the Microsoft SQL Server Transact-SQL Reference; for details on user-defined datatypes, see Using Datatypes to Enforce Data Integrity.