There are various references. The one I like is the following, particularly Remark 1.3.3.
MR2007376 (2005b:14049) Reviewed Abramovich, Dan(1-BOST); Corti, Alessio(4-CAMB); Vistoli, Angelo(I-BOLO) Twisted bundles and admissible covers. (English summary) Special issue in honor of Steven L. Kleiman. Comm. Algebra 31 (2003), no. 8, 3547–3618. 14H10 (14A20 14H30) http://arxiv.org/pdf/math/0106211.pdf