Ideology is difficult to define and probably therefore dangerous. David McLellan said "Ideology is the most elusive concept in the whole of social sciences". An ideology is one's collective set of beliefs or ideas, perhaps a philosophy. Ideology is rarely questioned by trusting devoted followers.
Ideology dictates the unconscious orientation, habits and social practices which may them have a strong influence on politics and government. An ideology is generally more strongly held than just an opinion, although the usage of the word "ideology" in the mainstream is loose about this.
Ideology assists those interested in visionary theory. Understanding people's belief systems allow another to more easily discuss and push an agenda of with the heavy use of the social engineering, psychology, propaganda, the mass media and other communication and cultural mechanisms to help create a specific society.
Terrorists have very strongly held ideologies, which incline some of them to die, or encourage other to die for their ideological cause.