While the phenomenon here called "categorism" is more commonly known either as "racism" or as "bigotry", such a wide definition is not the most common useage of either of these two words. It is more common to use each of these two words in more speciffic/narrow sense: Racism being categorism based on categorizing people into different races or skin-colors (a foci of categorism), bigotry being categorist emotional states (an expression of categorism).
"Prejudice" refers to unjustified beliefs beyond preconceptions, "bigotry" refer to emotional states such as hate/fear/supremacism, and "discrimination" refers to actions such as mistreating innocent individuals or implementing policies which causes mistreatment of innocent individuals. Prejudice, bigotry and discrimination against a category of people are the three archetypical expressions of categorism, which can be done in many different ways. For example by stigmatizing people or by exploiting them.
When categorism is done, it is done in a social context. This context can be interaction between two or a few individuals (micro level), a biger context such as an organization (meso level) or a very big context such as a country or global subculture (macro level). In the given context, a certain form of categorism may be more or less maginalized or more or less hegemonic. When certain people are stuck in a context where categorism against them (through a category they are identified as) is widespread or even hegemonic, it can be reasonable to count these people as being oppressed.
When people do categorism, they do it focused on a certain categorization of people. Such specific foci of categorism are commonly known by names such as Racism (when it's based on a categorization of "race", skin-color or ethnic background), Sexism (when it's based on a categorization of sex/gender) or Homophobia (when it's targeting homosexuals and to some extent other LGBTQ+ people). Note that the categorism model distinguishes between categorism and kyriarchy: Thus distinguishing between racism and colonialism as well as distinguishing between sexism and patriarchy. Categorism and kyriarchy are to a large extent intertwined, but they are not the same thing: While kyriarchy is systems of social/socioeconomic power (whether unfair/oppressive or otherwise), categorism is systems of prejudice/bigotry/discrimination (which any unfair/oppressive system requires to justify and perpetuate itself).