There are different types of diving that are both similar and uniquely different. Some people prefer one, some people enjoy both. You typically get certified in scuba diving as a stepping stone and as an intermediate step before becoming fully certified to open water dive.
Scuba diving certification takes about 2-3 days to complete, once you have completed the prerequisites associated with being able to get certified. In these 2 to three days, instructors and professionals will cover the fundamentals and the theory and concepts of diving as well as several pool sessions that will expose divers to different scenarios and allow them to test their skills. Finally, the 2-3 days will allow for a total of four sessions in the actual open water. When it comes time to get open water certified, it take longer, anywhere from four to seven days depending on the program and covers more thorough concepts, making a more in-depth and thorough certification. The structure and content is more thorough and allows divers to be able to dive and do excursions on their own whereas a scuba certificate requires a diver to do so with an instructor.