What is struct Input (Shader)?

Hi,

I was wondering what "struct Input" is ("tex2D" just needs for some reason to store the uvs, and it does it automatically with _MainTex in the first parameter?), and how do we know the "name" of what we define in the struct input (uv_MainTex etc.)? Is it an information we have to find in the Lambert shader?

For example :

 SubShader {
       Tags { "RenderType" = "Opaque" }
       CGPROGRAM
       #pragma surface surf Lambert
       struct Input {
           float2 uv_MainTex;
       };
       sampler2D _MainTex;
       void surf (Input IN, inout SurfaceOutput o) {
           o.Albedo = tex2D (_MainTex, IN.uv_MainTex).rgb;
       }
       ENDCG
     } 

EDIT :

in this example, viewDir takes the viewing angle from Unity... so what's inside "Input" is also directly connected to our properties outside the shader? I am a bit confused between the two examples :

     //...
     struct Input {
         float2 uv_MainTex;
         float2 uv_BumpTex;
         float3 viewDir; //We want to get the viewing angle
     };
     sampler2D _MainTex;
     sampler2D _BumpTex;
     float4 _RimColor;
     float _RimPower;
     void surf (Input IN, inout SurfaceOutput o) {
         o.Albedo = tex2D (_MainTex, IN.uv_MainTex).rgb;
         o.Normal = UnpackNormal (tex2D (_BumpTex, IN.uv_BumpTex));
         half rim = 1.0 - saturate(dot (normalize(IN.viewDir), o.Normal));
         //...

(source : http://cgcookie.com/unity/2012/11/26/introduction-to-surface-shaders-in-unity-part-0/)

Thanks