Yes, your tailbone is definitely different from your sitbones.

Your tailbone is your coccyx. Your sitbones are your ischial tuberosities.
http://www.nervemed.com/Piriformis/PyrAnatA108.jpg

You should be sitting on your ischial tuberosities. If you are not, try sucking in your abs and curling your pelvis under a little bit when you ride. Otherwise, perhaps you need a slightly wider saddle.