There's a huge difference. A PH is a compound that has to be converted before it becomes an active steroid. Some have their own intrinsic activity, but they generally lack any activity of their own. A steroid is an active hormone within the body and does not need to be converted.
For example:
M1,4ADD is a PH to dianabol that has ~15% conversion ratio
4AD is a PH to testosterone
Those are PHs that have to go through conversion before they really do anything. Now, the target steroid (e.g. test) needs no conversion before it is "active" within the body.