Understanding the Liturgical Worship Service: Acts of Entrance Part II

Confession

Explanation: Before we enter the presence of God we must be pure. Thus we confess, or acknowledge, to Him who and what we are: sinners. We are honest- knowing He already knows our sinful condition- but confident of His forgiveness and mercy.
Scripture: for the order, the LSB uses I John 1, Psalm 32, and Psalm 130. Concerning our need for confession: James 5 and Hebrews 10.
Sacramental (God has written His Law in the Bible as well as on our hearts) and Sacrificial (we confess our sin).
Alternatives: (People need to confess their sin; this portion of the service should not be optional, unless private confession is a regular occurance.) I find that most congregations that use their own words for the confession come up short. Either they are too generic (i.e., they tip-toe around the issue of sin) or they are too specific. (e.g. "Lord, forgive us when we eat too much turkey.") Creative Worship usually does a pretty good job of writing confessions that are well-rounded, but also reference the particular season.
Hymns: "Lord, to You I Make Confession" (608) The tune is unfamiliar to many, but well worth learning.

Absolution

Explanation: This is where the pastor, on behalf of Christ, announces to all that they are absolved, or forgiven of their sins. Many Protestants struggle with this, however Jesus says in Matthew 16:19, "I will give you the keys of the kingdom of heaven, and whatever you bind on earth shall be bound in heaven, and whatever you free on earth shall be freed in heaven," (translated by yours truly) and in John 20:23, "If you forgive the sins of anyone, their sins are forgiven; if you do not forgive them, they are not forgiven." (NIV)
Scripture: Matthew 16, and John 20.
Sacramental (God assures of His forgiveness in Christ)
Alternatives: Obviously, the Absolution must occur if there has been a confession. Again, check Creative Worship.
Hymns: Jesus Sinners Doth Receive (LSB 609) vv. 1,5-7

Introit (Latin for 'Entrance.')

Explanation: The Introit is usually a Psalm or a paraphrase of a Psalm, with a repeated refrain (Antiphon). Some chant the Psalm responsively, between the pastor and the congregation, others speak it. Once in a while a choir might sing a setting of the Psalm. Chanting is quite foreign to many people, but once learned it can be very effective in creating a numinous (having a wholly other quality) atmosphere.
Historically, this was the beginning of the DS, since Confession and Absolution was done privately. The priest, following the crucifer, et al., would chant the Introit as he made his way up the long cathedral. As the congregation realised what was going on (the bells and incense were big clues), they would turn and face the cross, reminding them that it is the cross that is the focus of God's service for humanity.
Scripture: Ephesians 5 and Colossians 3
Sacramental (God comes to dwell with/in us in His Word)
Alternatives: There are quite a few "contemporary" settings of Psalms out there.
Hymns: There are many paraphrases out there: our LSB has quite a few, the Evangelical Lutheran Hymnary (Evangelical Lutheran Synod) has a few more, and the Cantus Christi (Reformed) has a lot more, let alone Cyberhymnal. See also Hymnoglypt.