What's the difference between Omega-3 and DHA?

I noticed that with some prenatal vitamins some have the omega 3 pill with them and some contain DHA. Is there a difference or is DHA a form of Omega-3?|||Omega 3 and DHA are the same, just like Omega 6 and ARA are the same as well. Often it depends what country you're in. I think in USA they refer more to them as omega's whereas in Canada they are DHA and ARA.