From what I know - and that seems to be less and less each day
Is that
1.) Foster Child is supposed to be a temporary measure. If the child is over 14 they can choose not to be adopted - then there would be a guardianship.
2.) Gaurdianships are for older foster children or family members primarily. The do not make the child "safe" from the tpr'd bio's. If for some reason they bios turn everything around and can prove to be responsible they can go back and ask for custody again. I believe that is a risk with guardianships.
3) Adoption - is for all legal puposes now your legal child. The child gets a new name - usually your last name, sometimes first and middle all depends on what you do and then you get a new birth certificate...and they are legally yours - and have all rights as a child born to you. They will have rights to your estate when you die etc. etc. just as a bio child would.
So that is my understanding of the differences. I might be wrong - but that is my understanding at this time. Hope this helps.