Quality Assurance: It is nothing but building an adequate 
confidence in the customer that the developed software is 
accoding to requirements.
Entire SDLC comes under QA.
It is process oriented.

System Testing: It is the process of executing entire 
system ie checking the s/w as well as parts of system.

Quality Assurance:(1) The process of monitoring and 
improving the entire SDLC phases.
                   (2)The process of measuring the strength 
of SDLC(preventive action).
System Testing:system testing is nothing but testing the 
entire application with client requirement i.e with client 
data.

Quality assurance is some thing where in we validate the 
process i.e what is the process used to build a application 
like how many reviews are done and what are the phases 
involved. Quality control is some thing where in we 
validate the delivarables, we are not bothered about the 
process, we are bothered about the functionality, i.e how 
good is hte review done, how many review comments are 
given, are the review comments fixed and is the code 
working fine.

for a real time example:
consider preparing idly.

Quality assurance: here we see what is the process that is 
used to prepare idly like firt we have to prepare the 
batter, this invloves soaking of the ingredients for a 
while say one hour, clean the idly ravva and then grind the 
dal and mix it with ravva....here we are not botherd about 
the quality of dal and ravva and the quality of grinding,  
we are bothered about the proces
People assume that if we follow these steps we can assure 
that we can get idli

Quality control: here we are bothered about the quality of 
the outcome, we validate the quality of everything the 
quality of ravva, the qaulity of dal and the how well it is 
mixed all this stuff

control is a part of assurance.

Assurance starts from the design phase and control starts 
from execution phase