We are being required to start doing code reviews and I was wondering what the standard is for code reviews.

We have a QA department that tests our modifications and development from the front end so we minimize the bugs that get release in our product. Since we have a QA we've never done an official code review. If we see something in the code while we are modifying that section that we think is wrong we will fix that also.

Companies that do code reviews do they review every modification? Or do they only do it on certain criteria, like if a developer has less than 2 years experience?