First, you learn the rules (years of work), then you hear a Russian speaking in real life and don’t understand a single *^&% of it (just an example). Theory is always slightly different from practice, and sometimes the differences are significant.

A definition of slang is hard to pin down. But why try when others have already done it? According to Wikipedia, slang refers to “registers of language usage that are either reserved for a social group or considered ‘vulgar,’ meaning substandard language.”