From the US Catechism for Adults:
As we come to understand the Sacraments, it is important to recognize that the Sacraments have a visible and invisible reality, a reality open to all the human senses but grasped in its God-given depths with the eyes of faith. When parents hug their children, for example, the visible reality we see is the hug. The invisible reality the hug conveys is love. We cannot “see” the love the hug expresses, though sometimes we can see its nurturing effect in the child.
The visible reality we see in the Sacraments is their outward expression, the form they take, and the way in which they are administered and received. The invisible reality we cannot “see” is God’s grace, his gracious initiative in redeeming us through the death and Resurrection of his Son. His initiative is called grace because it is the free and loving gift by which he offers people a share in his life, and shows us his favor and will for our salvation. Our response to the grace of God’s initiative is itself a grace or gift from God by which we can imitate Christ in our daily lives. The saving words and deeds of Jesus Christ are the foundation of what he would communicate in the Sacraments through the ministers of the Church. Guided by the Holy Spirit, the Church recognizes the existence of Seven Sacraments instituted by the Lord.
For more information use the following links:
The US Catechism for Adults has an excellent section on Sacraments.
The US Bishops website also has a good section on Sacraments with additional links.
When he was at Milwaukee Bishop Hying developed a series of videos talking about aspects of our Faith. View his presentation of Why Observe the Sacraments.
BustedHalo is a website geared to young adults. They have a good selection of video clips in their Sacraments: 101 section.
The Catechism of the Catholic Church has a good but very detailed and somewhat technical treatment of the Sacraments. Note that this is from the Vatican website and isn’t the best display. There is an alternate version on the USCCB website but it doesn’t allow direct links to articles.