InputSplit is confusing me .. Any clarifications ??

[maha]

Hi  Everyone,

   What I really wish for Thanksgiving is some one giving me clarification of 
how the inputSplit is working. 

Eg. 

  public void map(LongWritable  key, Text value, OutputCollector<Text, Text> 
output, Reporter reporter) throws IOException {     
                  String line = value.toString();

  How did we know that map in this case is taking a line and not the whole 
input document ?

   
       Happy Thanksgiving everyone,

        Maha