Describing Individual types:

DESC keyword like describing other tables is used to DESCRIBE any object type. The following points describe the differences between Nested Table and Varrays:

• Nested tables are also singly dimensioned and a unbounded collections of homogenous elements. These are initially dense but can become sparse through deletions. In both PL/SQL and the database Nested tables are available (for instance, as a column in a table).
• VARRAYs, such as the other two collection types these are also singly dimensioned collections of homogenous elements. Therefore, they are always bounded and never sparse. Such as nested tables, they can be used in PL/SQL and in the database. Dissimilar nested tables, when you retrieve and store a VARRAY, its element order is avoided.

Using a nested table or VARRAY, you can retrieve and store non-atomic data in a single column. For instance, the employee table used through the HR department could store the date of birth for each employee's dependents in a single column.