When I learned the term, a racist was a person who believed that his/her own racial/ethnic/cultural group was innately superior to other racial/ethnic/cultural groups. Racism, then, was any behaviour that followed from that belief.
Actual racism needs a whole range of supporting nonsense to go with it: usually including pseudo-history, pseudo-science, pseudo-philosophy, and sometimes even pseudo-theology.
Actual racism, thankfully, is very rare.
Sometimes, however, we use the term racism when we mean bigotry. Bigotry is far more common than racism.
Bigotry is an irrational dislike of all the members of any particular racial, ethnic, religious, or cultural group, merely because they happen to be Jewish, or Aboriginal, or Muslim, or Catholic, or African, or gay, or Mormon ... or whatever. Usually bigots spend less time thinking about their bigotry than racists spend thinking about their racism. Bigots don't think the people they don't like are innately inferior, they just don't like them. Some bigots "catch" their bigotry from their parents, and may say things like "I don't like the Irish (...or whoever...) because my parents and grandparents didn't like the Irish."
But then there are some people who really don't have a racist or bigoted bone in their bodies, who get caught up in this as well. With a total absence of any ill will, they sometimes say absolutely stupid and offensive things about members of other racial, religious, ethnic, or cultural groups, and then are amazed when people take offense at the comment. They often defend themselves by saying, "But some of my best friends are Polish (... or whoever...) and they thought the joke was funny."
For this third group, their frequently offensive comments are part of a broader personal context of coarse language, adolescent humour, and crude behaviour.
- Crude racial terms take their place alongside other X-rated language.
- Racial or sectarian jokes take their place alongside "blue" jokes and fart jokes.