The ranks are separated generally into KYU (non-black belt) and DAN (black belt) levels.
Your school will typically have a syllabus of what techniques, at a minimum, you must be able to demonstrate to meet a certain rank. You can ask your sensei if he has this syllabus for your organization.
Taking promotional exams is important and not important at the same time.
It is important as it is part and parcel of the Aikido experience and it somewhat sets objective goals for you to try to achieve since there are minimum requirements set by your organization. But chasing belts for the sake of getting them will get you nowhere. Belt or no belt, think that you are in it for the long haul (ie lifetime). Achieving rank is not the challenge, the journey is.