SQL & PL/SQL

hi all,
please tell me difference between nested table vs associative array as per below statement.

1.Nested tables can be stored in a database column, but associative arrays cannot.
2.Associative arrays are appropriate for relatively small lookup tables where the collection can be constructed in memory each time a procedure is called or a package is initialized.

please make me comfortable with above points with examples so that i can actually understand both.


rgds,
pc