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