In the court context, a subpoena is a species of a court order but it's not the equivalent of an order by the court. Failure to respect a subpoena can give rise to a charge of contempt of court. In a similar fashion, a summons by a committee does not have the standing of an order of the House or an order of the court, but is a legal document and gives rise to legal obligations. Failure to comply with a summons could have legal consequences, but in the parliamentary context.
On November 27th, 2007. See this statement in context.