What is Baptism for the dead?

In I Corinthians 15:29 the people are being baptized for the dead.  Why?  What does this passage mean?

In I Corinthians 15:29 “Baptism for the dead” is a reference to being a martyr – dying for the benefit of another.  That is why Paul follows this statement with:  “And why stand we in jeopardy every hour?  I protest by your rejoicing which I have in Christ Jesus our Lord, I die daily.”  Paul is arguing, why would I put my life on the line if there was no resurrection from the dead.  Why be baptized into death for the spiritually dead if there is no resurrection. 

Baptism into death is what the Lord was referring to in Luke 12:50 and Mark 10:38.  Again, I hope this helps.