QA means Quality Assurance while QC means Quality 
Control... QC makes sure that correct procedures and 
standards are being followed in the company and by the 
employees... these procedures n standards can be of CMMI 
etc. or defined by the company...

QA got nothing to do with procedures and standards... 
his/her main task is to validate and verify projects.

QA Objective: 
- Evaluate the project process in order to identify the
defects so that they can be fixed.
- Determine the ways to prevent future defects in the
project process

S/W QA involves 
1) the entire software development process
2) monitoring the process
3) making sure that the agreed upon standards and procedures
are followed throughout project life cycle.

Audits and reviews can be done to monitor the process.

QC Objective:
Evaluate the work product in order to identify defects so
that the defects can be fixed and also to identify the
prevent the ways to prevent the future defects.
QC involves verifying the product with the requirement
specification.
It is done by executing software on computer.
Testing is the key technique used to perform product evaluation.