What is an Input in SDS-PAGE? - (Oct/25/2012 )

A lot of the times when I am reading a paper, the westernblot shows "Input" lane.. I don't understand what this represent. Please help me. I know its a noob question but Im only an undergrad. Thanks for your time.

Usually this will be where the gel has been run on samples that have been immunoprecipitated (IP). The input in this case is sample lysate that has not been IP'd and is used so as to show the level/presence of target protein(s) in the sample.