Adult guardianship (sometimes called conservatorship, depending on the state) gives one person legal authority to make decisions for another adult who's been found incapable of managing their own affairs — often the fallback when someone didn't set up a power of attorney before losing capacity.
Guardianship of the person vs. estate
Many states distinguish between guardianship of the person (medical and personal care decisions) and guardianship of the estate or conservatorship (financial decisions) — a petition can request one, the other, or both depending on the actual need.
The court process is more involved than a power of attorney
Unlike a power of attorney, which someone signs voluntarily in advance, guardianship requires a court proceeding, typically including a medical or psychological evaluation of the proposed ward and, often, appointment of an attorney to represent their interests.
Why advance planning avoids this entirely
A validly executed power of attorney and healthcare directive, signed while someone still has capacity, generally avoids the need for a court-supervised guardianship altogether — one of the strongest practical arguments for completing these documents well before they're needed.
Ongoing reporting obligations for guardians
Once appointed, a guardian typically has ongoing reporting obligations to the court — accountings, status reports — similar to an executor's duties, and can face personal liability for mismanaging the ward's affairs.