@Neonivek
Chaos is generally Chaos Space Marines, while Demons is chaos demons (I think there's only Khorne Daemonkin right now). They're battle brothers, just not the same faction. It's like how the Imperium isn't a single faction.
The current chaos armies with rules are Chaos Space Marines, Chaos Daemons, Khorne Daemonkin and Renegades & Heretics.
Chaos Space Marines is supposed to represent the Legions and warbands of the fallen marines specifically.
Chaos Daemons is the army containing the daemons of all four gods and is used for armies themed on daemonic incursions.
Khorne Daemonkin is for Khorne religious fanatics. The ones who seek to die in the name of calling his daemonic forces into battle.
Renegades and Heretics is the normal humans and mutants fallen to heresy and damnation, serving the dark gods with little to no hope of ever gaining their attentions.
Each army is separate but all can ally with each other as Battle Brothers the way the Imperium can. Chaos Daemons units cannot join or be joined by other units though because of a rules conflict with how they respond to damage compared to the other armies. Daemonkin units do not have this problem.
So basically there is a lot of infighting amongst all these groups.
Also incase anyone is wondering I am incredibly biased and love the Necron.
Would the Necron have any reason to infight?
Yes, the Necrons fight each other over dynastic rivalries thousands of years old, over territory and over more modern slights and grudges. Each dynasty has different goals from the others and this often puts them at odds.