What exactly is this series?

There is not "many versions"; in reality, it's one very long running series, splitted into multiple parts, each continuing the story of the previous one. Even so, the story told in the part itself can be understood without prior knowledge, and still is said to be good as a stand-alone one; which is part as to why many people liked the 2018 Netflix adaptation of New Grappler. It's somewhat similar to JoJo's Bizarre Adventures, if you know that series.

I'll still recommend you to read everything in order tho, as that's how the Author intended its reader to experience his works.

This one is the original series, New Grappler is the 2nd Part, Hanma the 3rd, Baki-dou the 4th, Baki-dou (2018) the 5th, and Rahen the currently ongoing 6th one. There is many Spin-Offs for each part, that you can find listed on each entry of MAL.