There are many different certification agencies. SDI, PADI, NAUI, YMCA, TDI, CMAS, NASDS, SSI. All of them follow the same basic principle which is to teach you to be a safe and comfortable diver. There are some basic prerequisites that must be met prior to enrolling in a SCUBA diving course. For ages 10 through 14 there are specific limits. They must dive with a parent or guardian and there are depth restrictions which are usually limited to 60 feet. Those in that age bracket receive what is called a junior certification upon completion of the course. Ages 15 and older receive what is called an Open Water Certification.
There are higher levels of certification but I will save that for another question.
The courses consist of academic, confined water training and open water training.
PADI Instructor Development Course