>>110609709What we know for sure is that he was a famous Kaleesh warlord who at some point might have fought the Republic in the past (before the clone wars), hence his vendetta against it. That's pretty much it. As to why he's a cyborg, there's actually some conflicting stuff.
The commonly known legends backstory (which I'm not sure is even canon anymore) was that he was horrible injured from his shuttle being shot down, hence his cybernetic enhancements. In the legends, it's Dooku who mostly manipulated events (including his crash) to essentially turn Grievous into his own pawn.
In the actual confirmed canon, however, it's heavily implied that most of Grevious's augmentations were intentional and gradual. Sort of like he would choose individual parts or cybernetics to help him become an even better warrior in combat. He just slowly morphed into a machine (pic related). Although the canon doesn't really delve into it, we just get hints and rumors towards his backstory.
