I was going to comment on the Brees/San Diego house thread, but this is a better place. :)
This is the way I understand it, and I've lived in all four corners of the US:
Bath/Full Bath: has bath tub
3/4 Bath: shower/no tub
1/2 Bath: no shower or tub
As far as I know, this is only an issue when you go to sell your house, or are assessed for property taxes.
Edit: And to comment on PayorPlay's post, I remember seeing the advertised number of baths as the total number, and then the written description would break it down into actual features.