When you're learning a new language, you'll meet some group of words that can be express as a single word in your mother tongue. If they are interchangeable, it's not a big deal, but in most cases, they have their own nuance and if you ignore it, the sentences become weird.
Words vs. Words web book series is for those who want to learn these different nuances. In every season, I've covered about 30 groups of words that can be expressed in English as a single word.