What is the up with the beef between Drake and Kendrick Lamar? And what about J. Cole?

I know they're posting diss tracks against each other, but I'm not sure why? What made this start? And how was J. Cole involved? And how has this feud evolved so far?

I feel so out of the loop on this one! All I see is reaction images of the songs and Kendrick destroying Drake, but without the context of why they were posted, I'm still confused.