A Sacrament is an outward sign of inward grace. So while it is plainly visible to all the senses, it's inner effect is all God's work. In fact, these special events amount to Christ acting in us, conferring his grace out of love for us his people. Hence, for example, Christ's command at the Last Supper to his disciples "do this in memory of me".
There are seven Sacraments:
Three are life-giving - Baptism, the Holy Eucharist & Confirmation
Two are healing - Reconciliation & the Anointing of the Sick
Two are of service - Holy Orders & Matrimony