Deindividuation, in plain English, is when individuals turn into a mob. Deindividualisation is when you paint a social group as a homogeneous slab whose members allegedly have no individual characteristics, like "Muslims", or "African-Americans", or "gays", or even "women".
Neither is a pretty thing. The herd instinct is behind Nazi rallies, lynching, and Black Friday stampedes. Ostracising social groups is Divide and Rule 101, an age-old technique of cynical mass control.
The two can cross-pollinate selectively. That's when it turns really ugly. Thus, London riots of 2011 became racialised as "ethnic minorities going out of hand", and the perpetrators get meted out 1800 years of prison sentences. On the other hand, drunk college jocks rampages or the Bullingdon club's violent antics are written off as "boys going wild". The class, race and increasingly religion-centred prejudice make out essentially the same events as if radically different in nature.
In essence, that's what anthropologists do too, just from the other (left-wing) side of the same paradigm. The psychological aspect of mob behaviour often escapes social scientists, who look for all answers in social contexts, as if individuals do not exist. Unfortunately for anthropologists, the tireless paeans of cultural difference, some things are just really "universal human nature". Dismissing that fact precludes any meaningful understanding of social events: humans are reduced to the reductive Homo Anthropologicus, a perennial cultural group actor, put into action by collectively shared beliefs and rituals differing based on class, gender, or ethnicity.