What does Baptism mean, why do it?

From Matt Slick's article on Baptism and Romans  6

"As Christ died and was raised to a new life, so too the Christian, in Christ, is said to have died (Rom. 6:11; Col. 3:3) and has a new life.  This new life of regeneration is by faith - the internal work.  Baptism is the external work of identification with Christ. This is why the reference to baptism in the Bible is dealing more with "our union and identification with Christ than to our water baptism."

- Baptism is being identified as a disciple (Matt. 28:18-9).
- Baptism may be compared to a new birth (John 3:5).
- Baptism is compared to Jesus' death and resurrection (Rom. 6:3-5).
- Baptism is compared to Israel's Exodus and passing through the Red Sea (1 Cor. 10:2).
- Baptism is compared to Noah's escaping the flood waters by entering the ark (1 Pet. 3:21).

In each of the references above, baptism is an identification with something.  When people were baptized into John the Baptist's baptism of repentance, it wasn't the baptism that granted them repentance or made repentance real.  Repentance is something that happens internally and is the work of God (2 Tim. 2:25)."