If you spar with someone, you exchange light blows — either literally by punching each other, or figuratively by exchanging verbal blows.
If you box, you might spar with an opponent at the gym while you're training. You don't strike too hard — it’s just practice. Similarly, if you spar with words, you typically do it playfully and without anger. You might repeatedly spar with your mom about who makes the best meatloaf, but it’s all in good fun. Spar is also the name of a rigging pole, as well as of some light minerals like calcite, that you can cut through.