A Job in Ragnarok is the class of character that you play. This determines your available skills and is a deciding factor in how you play your character. From a novice you can upgrade into a Swordsman, Thief, Acolyte, Merchant, Archer, or Magician.
In Ragnarok you get experience in one way; by killng monsters. Each monster kill awards you experience in both types of levels: Base levels and Job levels.
The base level roughly determines the power and experience of a character. As the base level of a character increases, he will gain additional points to distribute in one of 6 stats, each of which enhances the character in a different way.