Spark is used for some of the most powerful skills in the game
What are Chi and Spark? Chi and Spark are required sell maple story mesos to use certain skills, as indicated in their description. If you finish the level 9 cultivation quest, you can start collecting Chi. Any action, like healing, buffing and attacking, will get you Chi.
Chi is the yellow bar on your character name, as indicated here. When you finish the level 29 Cultivation, your Chi capacity rises from 99 to 199. 1 Spark represents 100 Chi. Spark is used for some of the most powerful skills in the game, so use it wisely.
What is Demon/Sage or Heaven/Hell? While doing the level 89 Cultivation quest, you have to decide whether to walk the Demon path or the Sage path.
When you're finished, your spiritual cultivation will rise to Celestial Demon/Sage depending on your choice. This has a few effects on you, most importantly; your skills will get an added effect, like a chance to restore some Chi, or added damage.
Demon is usually more fit for attacking classes, whereas Sage is aimed more at defense.
How do I change to a Tiger or Fox? At level 9, after finishing the Spiritual Cultivation for that level, you can learn the morphing skills, assuming you're a Barbarian or Venomancer.