Testing the code
The first question when discussing testing the code is a simple one: What exactly do we mean by testing the code?
While there are multiple answers to that, in the broadest sense, the answer could be "any procedure that probes the application to check that it works correctly before it reaches the final customers." In this sense, any formal or informal testing procedure will fulfil the definition.
The most relaxed approach, which is sometimes seen in small applications with one or two developers, is to not create specific tests but to do informal "full application runs" checking that a newly implemented feature works as expected.
This approach may work for small, simple applications, but the main problem is ensuring that older features remain stable.
But, for high-quality software that is big and complex enough, we need to be a bit more careful about the testing. So, let's try to come up with a more precise definition...