It is started by the filing of either a voluntary petition by the debtor[?], or an involuntary petition by creditors[?].
A stay of creditor actions against the debtor automatically goes into effect when the bankruptcy petition is filed. The stay provides a breathing spell for the debtor during which negotiations to resolve the financial situation can take place and a reorganization plan can be proposed.
See also:
wikipedia.org dumped 2003-03-17 with terodump