a derived table is dynamic,virtual table that you create
within universe structure.it consists of set of sql
statements that you create in designer and you can use it as
a logical table to create objects.

in the derived table sql set,you can put a select statement
in place of table name in the from clause.sql set can also
contain embeded prompts and you can create joins between
embeded physical tables ,just as you can with other tables.

derived tables can be used in place of database
views,aggregate tables,lookup tables.

by using derived table we are processing load from the
report server to the database.

views are also virtual tables which are created in database
level and can make use in universe.but we can not create
them in universe structure.