What Is Karma?

Karma is a Sanskrit word, but the word is also known in another form in a language called Pali. The scriptures that I'm talking from were written down in this language of Pali. This is to see the universe of some kind of a rewarder and punisher,. A rewarder for good conduct and a punisher for bad. If there is no soul, what is reborn? Thirdly, is rebirth true, and is it to be taken literally or metaphorically? Finally, in what ways is karma a useful concept or idea?