2d texture access using tex2d or tex3d

Hi,
I have a 2d matrix texture and I am not sure what is the diff in accessing it using tex2d or tex3d. Of course if I plan to access it using tex2d I will set the depth to 0 and for tex3d the depth to 1.
But assuming that the setup of texture is correct, is there a diff in using tex2d or tex2d?
Thanks,
G