What is Baptism?

For if we have been united together in the likeness of His death, certainly we also shall be in the likeness of His resurrection,  knowing this, that our old man was crucified with Him, that the body of sin might be done away with, that we should no longer be slaves of sin.  For he who has died has been freed from sin.  Now if we died with Christ, we believe that we shall also live with Him,  knowing that Christ, having been raised from the dead, dies no more. Death no longer has dominion over Him. For the death that He died, He died to sin once for all; but the life that He lives, He lives to God.  Likewise you also, reckon yourselves to be dead indeed to sin, but alive to God in Christ Jesus our Lord.(Romans 6:5-11)

So what is baptism anyway? If it required? Is it needed? Is not being baptized disobedience? Churches and various doctrines differ. Obviously, baptism does not save a person from sin, or those baptized by John would have not needed a Savior and the good thief would be in hell, making Jesus a liar.

However, baptism was important enough to Jesus that He gave us these final words of instruction:

And Jesus came and spoke to them, saying, “All authority has been given to Me in heaven and on earth.  Go therefore and make disciples of all the nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all things that I have commanded you; and lo, I am with you always, even to the end of the age.” Amen. (Matthew 28:18-20)

You decide. What is  baptism? Is it required? Is it disobedience if one is refusing to be baptized or if we do not require it for proper fellowship among believers?