Kitso... this thread's for you! But I'll do my best as an unwashed heathen (I'm an alum of the University of Oklahoma) to explain the hallowed traditions of Aggieland:
Aggies remain standing throughout the game; the yell leaders (there are no cheerleaders at A&M) lead the crowd in various A&M cheers. Most Aggies will know what cheers they are based on certain hand signals given by the yell leaders.
The 12th Man symbolically is represented by the assembled crowd of Aggie fans in Kyle Field.
More info on the traditions of Aggieland at
http://aggietraditions.tamu.edu/ .