Racism is when you divide groups between Class , Ethnic , Religion , Skin Color , National etc.
For example
1.You are from lower Class , you are poor and you have no rights to step in my door because of that.
2.You have different nation identity from my own nation identity so you have no rights to interfere in my Country or Business or whatever,everything that happen today here is your fault so i will point finger and blaming you , only because you are here or you are born here you have no rights at all and everything that happens bad is your fault no matter if problem is from something else or not.
3.You are from another Religion and everything that happen today is because people from your Religion creates Terror , therefore because you are from that Religion i consider you as Terrorist or "unbeliever" or "believes in devil" no matter if you are good or bad or Human Being.
4.I am White and you are Black or Yellow , therefore you are not welcome here because i treat you like you don't belong here no matter if you are born in Earth same as i do.
5.I consider "nation name" as my own , you have no rights to call yourself like that neither you are available to call yourself like that , you are "fake" created and you have no rights at all.
That was the 5 Examples with Racism in practice but there are many many others too.
Racism is known among scientists as "Modern Slavery" and is act of Terror and is extremely Discriminating and Dangerous that violate all the Human Rights.
Today in World of Warcraft Vanilla we can see some similar actions that are everywhere present such as example : You are Retribution Paladin and you have no rights to join in my Raid (PVE) no matter if you are PVE spec or not, whether you preform better than others or not, whether you contribute better than others or not , your Spec is not welcome because of that,this,there,here. Go reroll Holy or Mage or Warrior if you want to join my Raid.
Song that will inspire everyone against Racism , feel free to watch the Video as whole :
https://www.youtube.com/watch?v=PibXTko6VC4