design top down vs bottom up code example

Example: bottom up approach vs top down

Testing takes place from top to bottom. 
High-level modules are tested first and
then low-level modules and finally 
integrating the low-level modules to
a high level to ensure the system is
working as intended. Stubs are used as
a temporary module if a module is 
not ready for integration testing.

It is opposite of the Top-Down Approach. 
Testing takes place from bottom to up. 
Lowest level modules are tested first
and then high-level modules and finally
integrating the high-level modules to a
low level to ensure the system is 
working as intended.  Drivers are used
as a temporary module for integration testing.