This is a bit like asking: How is a flower different from a plant? Hooking is a technique that can be used for code injection. But not all techniques for code injection are hooks. For example, buffer overflow bugs or specially crafted malicious input data can be exploited to inject code, yet they are not hooks. (Also: https://en.wikipedia.org/wiki/Hooking , https://en.wikipedia.org/wiki/Code_injection)

Also, all hooks are not used for code injection.

Since words often mean different things in different contexts, please be sure to clearly convey the context you are asking about: What does SD stand for? Service Design?

@meriton (On the Software Development site, can we assume that's what SD stands for?)

@elgonzo, I think your comment should be an answer ; you might even want to sum some of it as "every hook is a code injection but not every code injection is a hook".