Scope of practice. That is the key, but it varies by state.
Here in this area, LPNs can not do patient assessments, they have to be done by an RN. They can not start IVs, hang chemotherapy (sp?)). They also can not do central lines, or central line dressing changes. In many areas they can not receive certifications, such as TNCC, or CEN, or practice in the OR, ICU, L&D, or ER. The ANA does not classify them as "professional nurses", and dont not allow them to join. The term "professional nurse" only applies to a RN. (No flaming! As this was taken from the Nursing Practice Act of Missouri).
The best way to find out the differences is to get a copy of your state's Nursing Practice Act. Keep in mind that the practice acts state the "largest" scope of practice, but it can be limited further by the employing institution, but it can not be added to.
Does that help?
And I mean no offense to LPNs, as they work their butts off too, but there are legal differences.