When is a court opinion considered precedent?

All published opinions (majority, concurring, dissenting, etc.) except per curiam (unsigned opinions) may be cited as precedent. Supreme Court opinions supersede all lower court opinions, and set binding precedents which both federal and state courts* are supposed to adhere to under the doctrine of stare decisis.

* US Supreme Court decisions only apply to state courts if they involve incorporated parts of the Bill of Rights or other applicable amendments.