737-600s, 737-700s, 737-800s and 737-900s are part of the 737NG/Next Generation series that debuted in 1998. They are still flying and are safe. 737-8s are part of the 737MAX series (which includes 737-7s, 737-8s, 737-9s, and 737-200s), and they are all unsafe/grounded. As far as I know, the 737-8 is the only MAX variant to actually have been delivered so far, but I can't find a proof of that at the moment.