QA is Quality Assuarance, which is responsible for 
improving the process of making an quality product. This 
involves all the phases of SDLC. 

QC is Quality control, which is used to improve the quality 
of product. This can be achieved by detecting and fixing 
the bugs.

Any activity of QA is assuring the Quality of the specific
product,which is already achieved at certain level.Once the
level of quality is ensured,QA will suggest the ways to
improve it to next hight levels of quality using established
QMS methodologies, as per the product development progress
and time to market strategies.

Whereas QC is, once the accepted level of Quality is
achieved, QC activity maintains it using the controlling
techniques to sustain at the same level for certain period
of time.As the durability prolongs at definite intervals of
time,QC measures the quality of the product to control the
accepted level of quality.QC activity is more of periodic
checks and implementation of methods to controlling the
product quality through tools and techniques.

Padmas, Good one...

QA - Quality Assurance is a set of activities that will be 
performed to ensure the product/process is in compliance 
with the standards defined.

QC - Quality Control is an activity to ensure that the 
product is meeting the requirements.