Chapters that have a codex will have chapter specific traits. For example: Grey Knights are Space Marines, but have different traits/abilities than Ultra Marines. Far as fluff goes, each has their own and all comes down to what you like/don't like. The same goes for look. This also leaves you open to create your OWN chapter should you choose to go that route.
The reason (since you said you didn't read into the fluff yet) for so many Space marine variations is because after the Horus Heresy it was decided that (and I'm over simplifying here) the chapters shouldn't have 10,000 Marines to draw upon or be that much of a threat should they turn against the throne. So they split each chapter into smaller, new chapters. 10,000= 1 chapter, now it's 1,000 per chapter so 10,000= 10 chapters (again, over simplifying).
If you want more detail to your questions, let us know. If you have more specific questions, fire away.
Welcome to Heresy!