There are many types of retreats, but the Spiritual Exercises of St. Ignatius of Loyola are unique. They represent a formal pattern based on four "weeks" of meditations beginning with God's love for us and our own attachment to sin that hinders us from responding. Most of the retreat involves meditating on the life of Christ from the Incarnation to the passion to the resurrection. Each "week" involves praying for a particular grace -- for example the grace of sorrow and tears as we reflect on Christ's passion and what our sins cost the Lord of the Universe, and the grace of great joy at our Salvation as we ponder the resurrection and Christ's opening the gates of heaven. You can read more about the Spiritual Exercises here. And if you are interested in more there's a wealth of information here. I hope you go browsing to learn more about this great spiritual method of growing closer to the Lord through meditation and contemplation on the life of Christ. St. Ignatius, pray for us.