Korean howitzer that Poland will buy. What is K9?

A heated discussion is caused by the Polish Ministry of National Defense's planned acquisition of the Korean self-propelled cannon howitzer K9, which is partially the progenitor of our Crab. Regardless of the discussion about the validity of its wide introduction in a situation where Poland produces the Crab (with K9 chassis), it is worth answering questions what is this construction, what is its past and how it might look in the future?