Here's why:
* Adverbs modify verbs, adjectives, or other adverbs.
* Example: "He effectively communicated his ideas." (modifying the verb "communicated")
* They tell us how, when, where, or to what extent something happens.
* "Effectively" tells us the manner in which something is done.
Let me know if you have any other words you'd like to know the part of speech for!