Welcome to Code Review, we're glad you found this site and have shown interest!
Even though Stack Overflow and Code Review are both focused primarily on code, there is a marked difference that should help you choose which site is best for what purpose. Many Code Review users also frequent Stack Overflow, either to ask questions or provide answers.
Here is the short version:
Stack Overflow addresses specific problems.
Questions that have a clear description of the problem, as well as an explanation of what you tried, etc. are usually well received. Questions with unclear, vague problem statements or questions that show little effort on the part of the asker are usually not well received. Stack Overflow prefer to have the minimum amount of code required to reproduce the problem.
It is also acceptable to post questions about how to write code, as long as you can show that you have a specific problem and what you have tried so far, where you got stuck, etc.
Code Review addresses improvements on all aspects of code which already works as intended.
On Code Review, questions that are well received provide real, working code, along with a plain English explanation of what the code does and why, and the more background information, the better.
On CR, it is fairly common to see posts with dozens, sometimes even hundreds of lines of working code, which on Stack Overflow would get down-voted, but on Code Review is welcome.
I hope this helps clarify the difference between the sites!