If you've taken the BACB-approved course sequence within your program (see www.bacb.com for schools/programs), once you graduate you can sit for the exam to become a board certified behavior analyst (BCBA). Most likely, with a masters in ABA and a BCBA certification, you may find jobs in public or private schools, in private agencies, hospitals, etc., as an ABA supervisor or provider. The population you would most likely work with at this point would be people with developmental disabilities. However, the field of ABA is growing in other areas, including dementia and traumatic brain injuries, to name a couple. I'm a BCBA, and from what I've seen, at least in Southern CA, it's mostly autism/developmental disabilities.
Now, there is another, nonclinical side to the field of ABA. However, most masters degrees in ABA are more focused on special ed, so if you're looking to work with rats and pigeons, or if you're looking to get to the philosophical and experimental nuts and bolts or even other applications that are as yet still developing, you will probably need to pursue a PhD.
I hope that helps!
