The basic diff is in unit testing the componenta are
replaced with the stubs and drivers wheres in the component
testing these stubs and drivers are replaced with the real
module.

Plz correct me if iam wrong.

System Testing : Soon after the integration testing is over
the application is deployed into the logical container (real
environment) then it becomes system if one performs testing
on these system then it is called system testing.

Component Testing : Individual units(Entities) are combined
to form an entire module if one perform testint on thes
module then it is called module testing or component testing.

In the System Testing apart from GUI and Functionality
testing mainly concentrate on performance part of the
application that may include Load Testing, stress testing,
Data-Volume testings are performed at this stage.
But in component testing mainly concentrate on functional
part of that application.