In the Catholic Church, justification (from the Latin iustus, righteous; and facio, to make) means what God does in order to render people righteous. (In theological lingo, justice means righteousness; a just person is right with God.) As the Catechism of the Catholic Church [CCC] puts it,
The grace of the Holy Spirit has the power to justify us, that is, to cleanse us from our sins and to communicate to us “the righteousness of God through faith in Jesus Christ” and through Baptism (No. 1987).
Justification, therefore, is entirely the work of God (see also the Decree concerning Justification of the Council of Trent, Canon 1), although, of course, it requires a free acceptance on the part of the person:
Justification is at the same time the acceptance of God's righteousness through faith in Jesus Christ. Righteousness (or "justice") here means the rectitude of divine love. With justification, faith, hope, and charity are poured into our hearts, and obedience to the divine will is granted us (CCC 1991; see also the Decree concerning Justification Canon 4).
Justification is the work of grace:
Our justification comes from the grace of God. Grace is favor, the free and undeserved help that God gives us to respond to his call to become children of God, adoptive sons, partakers of the divine nature and of eternal life (CCC 1996).
For most Catholic theologians, salvation refers to eternal salvation, that is, people’s full and definitive union with God. For example, that is the use given in the Decree concerning Justification.
Salvation does not, therefore, coincide perfectly with justification. Justification is absolutely necessary for (eternal) salvation; therefore, justification could be viewed as the beginning of salvation. (See CCC 1992.)
Justification, however, at least while people are here on earth, can never be considered definitive, since people can always lose their righteousness by deliberately committing grave sins. (See Canon 23 of the Decree concerning Justification.)
To sum up the difference: justification means what God does to make people righteous and right with Him; salvation means the full realization of that justification, when people reach their definitive union with Him.