Thread: What is what in a 4*4 matrix

I am kinda new to this so please bare(sp) with me
When I describe the orientation of something I am supposed to use a 4*4 array aka matrix..right? But what does each field represent in that matrix? This is kinda where I am now. Each row represent a vector and the lenght of that vector is always 1. But what does the last row represent. Links are highly appriciated. Is the first row always representing the "models" X values second row Y etc...?

Code:

dXX dYX dZX  Rotation aroundX (A)?
dXY dYY dZY  Rotation aroundY (B)?
dXZ dYZ dZZ  Rotation aroundZ (C)?
?   ?   ?    ?

~Barjor