Liturgy is the means that Christ has given us, through His Church, to worship God, to instruct us, to sanctify us and to unite us more to Him. The Liturgy includes the Mass, all the sacraments, the Liturgy of the Hours, and any liturgy, for ex. Liturgy of the Word or liturgy of Good Friday (neither are a Mass because there is no Consecration). But when we talk about the liturgy here we are normally referring to the Mass, Sacraments (like Confession), and the Liturgy of the Hours.
The earthly liturgy is a small imitation and participation in the the heavenly liturgy and prepares us for the eternal worship of God that will never end! It is God Himself who acts, the Father in the Son who in the Holy Spirit acts through the Church who offers true worship, worship in spirit and in truth to the Father.
The liturgy is the thirst that Jesus has to glorify His Father. It is the source of interior life and spring of apostolate. It unites me to the Church, it's members and its Life. It is real and active. It’s a gift I should not despise because it facilitates the formation of my soul in virtue. It is how I can “put on Christ” and Christ crucified. And it favors the permanence of the supernatural in all my actions.