IMAX or 3D only? What is the Diff?

I have been asking myself this for days now and couldn't get a clue what is the difference between Imax and 3D only cinemas. I have tried watching Imax yeah it was great and lively but what is the difference when you say 3D only? Is it the color? or the effects? I also notice Imax movies here are costly than 3D movies about 4-5$ up. Probably just here In the Philippines.