Help] Shader introduction

//*****************************************************************************
// TSE -- water shader                                               
//*****************************************************************************
//-----------------------------------------------------------------------------
// Structures                                                                  
//-----------------------------------------------------------------------------
#define IN_HLSL
#include "..\shdrConsts.h"

struct VertData
{
   float4 position        : POSITION; // Question: Is this the position of where the water block is? 
                                                      //   If so, what is a complete listing of such variables which are 
                                                      //   taken from the game itself?
};


struct ConnectData
{
   float4 hpos             : POSITION; // hPos = Horizontal position?
   float4 texCoord         : TEXCOORD0;
   float3 lightVec         : TEXCOORD1;
   float2 texCoord2        : TEXCOORD2;
   float4 texCoord3        : TEXCOORD3; // Question: All of these TEXCOORDS I don't understand Texture 
                                                             //   Coordinates of what?
   float2 fogCoord         : TEXCOORD4;
   float3 pos              : TEXCOORD6;
   float3 eyePos           : TEXCOORD7;
};


//-----------------------------------------------------------------------------
// Main                                                                        
//-----------------------------------------------------------------------------
ConnectData main( VertData IN,
                  uniform float4x4 modelview       : register(C0),
                  uniform float3x3 cubeTrans       : register(VC_CUBE_TRANS),
                  uniform float3   cubeEyePos      : register(VC_CUBE_EYE_POS),
                  uniform float3   eyePos          : register(VC_EYE_POS),
                  uniform float3   lightVec        : register(VC_LIGHT_DIR1),
                  uniform float3   fogData         : register(VC_FOGDATA),
                  uniform float4x4 objTrans        : register(VC_OBJ_TRANS),
                  uniform float4   waveData[4]     : register(C50),
                  uniform float4   timeData        : register(C54),
                  uniform float2   waveTexScale[4] : register(C55),
                  uniform float    reflectTexSize  : register(C59)
)
// Question: All of the above (ConnectData) I assume is the game, connecting shader data, with data taken 
//  from the game? If so, what does the register function do, What arguments does it take (What does C0, 
//  C54,55,and59 mean?

{
   ConnectData OUT; // Telling it to output this data back to the game?
   OUT.hpos = mul(modelview, IN.position); // Question: Mul? Mule?

   // set up tex coordinates for the 3 interacting normal maps
   OUT.texCoord.xy = IN.position.xy * waveTexScale[0];
   OUT.texCoord.xy += waveData[0].xy * timeData[0];

   OUT.texCoord.zw = IN.position.xy * waveTexScale[1];
   OUT.texCoord.zw += waveData[1].xy * timeData[1];

   OUT.texCoord2.xy = IN.position.xy * waveTexScale[2];
   OUT.texCoord2.xy += waveData[2].xy * timeData[2];

   // send misc data to pixel shaders
   OUT.pos = IN.position;
   OUT.eyePos = eyePos;
   OUT.lightVec = -lightVec;
   
   // use projection matrix for reflection / refraction texture coords
   float4x4 texGen = { 0.5,  0.0,  0.0,  0.5 + 0.5 / reflectTexSize, // Question: What is a float4x4 datatype?
                       0.0, -0.5,  0.0,  0.5 + 1.0 / reflectTexSize,
                       0.0,  0.0,  1.0,  0.0,
                       0.0,  0.0,  0.0,  1.0 };

   OUT.texCoord3 = mul( texGen, OUT.hpos );


   // fog setup - may want to make this per-pixel
   float3 transPos = mul( objTrans, IN.position );
   OUT.fogCoord.x = 1.0 - ( distance( IN.position, eyePos ) / fogData.z );
   OUT.fogCoord.y = (transPos.z - fogData.x) * fogData.y;
   
   return OUT;

}