Being a racist is a belief. You can just think it because of how you were raised, without really considering it very carefully or even realizing it's what you think, and without really knowing how to discuss or defend it. Probably everybody in America believes some racist stuff.
Being a white supremacist is an ideology. It has a coherent, internally consistent belief structure, usually based on Mein Kampf, that provides a rationale for action and for fitting ongoing events into your historical narrative. It has a catechism for mentally managing challenges to your belief and initiating new believers.
People are often just racist. The internet is where they learn (or are often lured unwittingly into believing) that they have a choice between the mainstream ideologies, most of which accept the cosmopolitan agreement that racism is morally wrong and a sin to overcome, or an alternate ideology, in which it is a virtue to be exercised.