This article is from the Copyright Law FAQ, by Terry Carroll with numerous contributions by others.
It's always at least a civil matter (a tort). 17 U.S.C. 501(b) details
the mechanisms by which an owner of a copyright may file a civil suit,
and 28 U.S.C. 1338 expressly refers to civil actions arising under the
However, under certain circumstances, it may also be a federal crime. A
copyright infringement is subject to criminal prosecution if infringement
is willful and for purposes of commercial advantage or private financial
gain. 17 U.S.C. 506(a). If the offense consists of the reproduction or
distribution, during any 180-day period, of 10 or more copies having a
retail value of more than $2,500, the offense is a felony; otherwise, the
offense is a misdemeanor. 18 U.S.C. 2319.
As a side note, although 18 U.S.C. 2319 purports to prescribe the
penalties for criminal infringement, all crimes covered by Title 18 have
their penalties determined by the U.S. Sentencing Guidelines, another
part of Title 18.