I work - means I have a job I'm working out - means I am exercising I work on - not complete - have to work on something. "I work on improving my pronunciation"

I work--this is just the basic personal use of the verb. (travaille?) "I work hard but I still have bills." I'm working out--this specifically refers to exercise. "I'll be a little late for dinner, I'm working out tonight." I work on--this is used if you want to describe the specific activity that you work on. "I work on my model airplane every weekend." "I work on my boxing but it's slow progress."

I work = you're a person who has a job. You're not retired, you're not unemployed, etc. I'm working out = to exercise, like the others mentioned. You're at the gym lifting weights. I'm working on = not a complete sentence, but would be used to mean that you're working on a specific thing. Example: James works as an architect. Currently, he's working on a project to build a house. Jane works as an attorney. Right now, she's working on the contracts for James' house project. After work, James likes to work out in the gym, and Jane likes to work out at home.