A brief is a written argument submitted by a party to an appeal that explains why that party should win. An opinion is a written statement that explains the reasons for the Court's decision in a case. Not all cases are decided by a written opinion, and not all opinions are published decisions of the Court that can be relied upon in other cases.
The State Law Library scans and posts all Montana Supreme Court opinions and the briefs that were submitted for those cases. This site provides copies of opinions and briefs dating back to the 1980s.