[Tron's expression darkens as the other asks for more information about Clu.]

Clu is the system admin program who decided that he could run the system better than his own User. He tried to kill him and took over... he appropriates programs for his own army, "rectifies" them, forces them to follow his will.

[He's going to just leave out the part where Clu did the same to him... and who Rinzler really is.]

He's dangerous and powerful, and not someone to go up against without careful planning.