As IRStuff's reply but...
If you have one in place and dont use it after your auditor has given you a certificate, you have "Marketing".
It bothers me greatly how many companies have an ISO9001 certificate in their offices but only pay it lip service when the auditor comes around.
Yeah, but that's ALWAYS been the case, even before the 3rd party certifications. All the blah, blah, blah about "world-class" and "highest standards," been there, done that.
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!
To my long experience ISO 9001 only assures consistency in you products therefore, if your work is good all products will be good. However, if you do a lousy job all your products will the same. ISO 9001 forces you to work to strict and fixed documented design and development procedures, work and test procedures. However, the catch is that you and only your professional are the people who writes the documents and procedures. No one else know better what you design how you design and make. Therefore, if your company workers are professionals, committed and caring, the design and and products quality will be OK. However, if your company doesn't value professionals and experienced staff but see everything in Dollars through shortcuts in the R&D, design and testing in the development process (which is very easy to save by cutting corners, less prototypes, less design iterations, less time for the professionals to think, analyse, research, etc.,) then your ISO 9001 will be your downfall.
ISO 9001 is a standard to establish, maintain and improve a Quality Management System (QMS).
Quality Assurance is anything you do to verify (or assure) that whatever you claim you are doing, is actually being done.
So, if let's assume your company X is having contracts to provide products or services to companies Y & Z.
If your contract with company Y says that you have to have and implement an ISO 9001 QMS, then quality assurance can be the audits performed to ensure that.
If your contract with company Z does not mandate you to have an ISO 9001 QMS, then quality assurance will have nothing to do with ISO 9001. You might still have to carry out audits for your own company (or even for the contract), and you can even use ISO 9001 as a guide, but not mandatory.