Like practically any language, Russian is rich in slang. Russian slang is a whole big world with equivalents for most words comprising the standard Russian vocabulary.
One of the biggest and most dynamic parts of Russian slang is that of teenagers. In teenage slang, new words can appear overnight, and with enough virility, they catch on and stay.
The best way to learn Russian slang is by listening to native speakers of Russian in informal conversation. It is also a good idea to ask native speakers about the meanings of words before adding them to your vocabulary—even words that look safe at first sight can have additional shades making their use inappropriate in certain situations.
While slang undoubtedly makes casual speech more natural, one should be careful about using too much of it. Ideally, a learner of Russian should be able to replace a slang word with a neutral one if need be, which requires a good command of Russian and a well-developed vocabulary. So our advice is you should not focus on Russian slang too much before you know enough standard Russian as well as always take caution when using Russian slang in serious conversation.