A Guardian of Property is someone who a court or the Office of the Public Guardian and Trustee appoints to manage your money and property. This might happen if you're not mentally capable of doing this. Or, if the attorney you named in your Power of Attorney for Property is not looking after your money and property properly.
A Guardian of the Person is someone a court appoints to make personal decisions for you if you're not mentally capable of making them. Their role is to make the decisions you would make for yourself. If the Guardian does not know what you would want, they must make decisions based on what they believe is best for you.