In system testing,Test Engineer will be testing the whole 
application without knowing the inner design of code or 
logic of the application.

In System Testing the tester will also conduct the Compact
ability test, Performance Testing, Security testing ,load
testing,volume testing etc

Actually System testing means to test whole the 
application.it has 2 types 1)Functional and 2)NOn 
functional.in functional testing tester need to test the 
application as per SRS.he needs to perform each function 
and tally the actual result with expected.and then he needs 
to do non functional testing.that means other than SRS.in 
this testing type tester should be perform testing as per 
users point of view.there r some types of non functional 
testing that is=>1)Usability testing
2)Accessibility testing
3)GUI testing
4)Performance testing(Load and Stress)
5)Volum Testing
6)Compatibility testing
each testing should be performed atleast once.

System testing is the combination of Functional testing ,Non
functional testing and performance testing.

Functional testing involves:

Smoke testing/sanity testing
User interface testing
Error handling testing

Non functional testing:

Reliability testing
Compatibility testing
Configuration testing
Inter system testing
Data volume testing
localization/globalization testing
Scope testing
Scalability testing
Multi languish testing
Security testing

Performance testing :

Stress testing
Load testing
Endurance testing