A Power of Attorney has the authority to handle your property and finances. They are also able to sign for you while you are still living. However, when you pass away, this power also ends.
On the other hand, an executor for your Will has the authority to handle your property and finances after you have passed away.
So basically, power of attorney has authority to act on your behalf while you are alive while an executor has the authority to act on your behalf once you have passed away.