When you use it as a verb or action, you should use "work". For example: "I have to work today". In this sentence work is the action. If you said, "I have to job today", that would be incorrect. If you use it as a noun, you can use both "job" and "work". Example: "I have to go to my job". In this sentence job is referring to a place or physical destination. "I have work to do." In this sentence the word job is a thing or an idea. Work can be a noun or verb. Job is used more as a noun. I don't want to confuse you but you can use jobbing as a verb/action but I don't think a lot of people use it that way. =)

In this case the two words are interchangeable.