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.
Brandy is pretty much on track with her post but I'd like to add that here in Ohio and in PA, LPN's can and do work OR and L&D. When I worked at a hospital in Erie, PA, that facility would not allow LPN's to hang blood, do anything with ppn or tpn or give any medication IVPush except for routine saline lock flushes. Also at that facility the LPN's pt's had to be co-assigned to an RN as they were not allowed to accept phone orders from doctors and I as an RN had to co-sign their notes in the patient charts. That was that specific facility's policy. Another hospital in that same town did not require that the LPN's ahve notes co-signed by an RN or have an RN cover their patients.
There seems to be no difference between what LPNs and RNs can do in my hospital. LPNs do assessments, hang blood, take orders, start IVs, give IV meds, just like RNs do. When assignments are made, no one even checks to see who has what initials after her name. I was oriented for a time by one of the LPNs. LPNs work in every area of the hospital, as well.
Before I went to school, I was an NA/monitor tech at another small hospital in a neighboring state. There LPNs had to have an RN cosign orders, could not do admission assessments or IV drugs. I guess it all depends on where you are.
LPNs can work anywhere in the hospital that the hospital is willing to employ them. When I worked in ICU, they couldn't push IV meds, wedge a Swan, or shoot a cardiac output, but they could and did do everything else. A lot of hospitals forced their LPNs out, and now they're in a bind due to the nursing shortage, and having to let some of them back in.
LTC facilities couldn't survive without LPNs. I have some terrific LPNs on my unit, who are indistinguishable in their practice from the RNs. But it's a pain that they're not allowed to supervise any more.