...but what IS it really?

In baptism, the Word of God says we are baptized into the Lord's death.

Are you ignorant of the fact that all of us who have been baptized into Christ Jesus were baptized into His death?
Romans 6:3

...burial

Therefore we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life.
Romans 6:4

...and resurrection

For if we have been planted together in the likeness of his death, we shall be also in the likeness of his resurrection:
Romans 6:5

Even the act of baptizing someone (full immersion) resembles all of this scriptural truth. Going down into the water (death), going under the water (burial), coming out of the water (resurrection).

There are many doctrinal arguments as to whether someone should be baptized in the Name of Jesus or in the Name of the Father, Son and Holy Spirit. I think these kinds of differences between believers is neither pleasing to God nor beneficial to the Body. Albeit, i offer the following scriptural references:

Acts 2:38
Acts 8:12
Acts 10:48
Acts 19:5

In water baptism, we recieve the Name of the Lord Jesus Christ and are clothed in Him.

For as many [of you] as were baptized into Christ [into a spiritual union and communion with Christ, the Anointed One, the Messiah] have put on (clothed yourselves with) Christ.
Galatians 3:27 AMP

A New Testament believer enters into a covenant relationship, which becomes tangible through water baptism. As the "old man" is cut away and buried, we experience a new life in Christ.