Adverb, as defined by Oxford Dictionaries:
1. a word or phrase that modifies or qualifies an adjective, verb, or other adverb or a word group, expressing a relation of place, time, circumstance, manner, cause, degree, etc. (e.g., gently, quite, then, there.)
I’ll make it easier. Many, though not all, adverbs end with an -ly suffix. Often they describe an action.
Consider the following two passages.
Jake looked into Macy’s eyes.
She looked back.
He took her hand and raised it to his lips.
Now, let’s add a few adverbs.
Jake looked passionately into Macy’s eyes.
She looked shyly back.
He gently took her hand and raised it slowly to his lips.
Voila! The description goes from sounding like a Dick and Jane reading primer to something a romance fan might want to read.
Adverbs. They’re your allies. Sprinkle them liberally throughout your writing.
Seriously? Adverbs are (not) your allies.
Subverting Adverbs and Clichés by Chuck Sambuchino via “Writer’s Digest”
“Five Habits to Avoid in Fiction Writing” from Scribendi — Guess what? One bad habit is the use of too many adverbs.
“Use of Adverbs in Fiction Writing” by DarcKnyt via “Deviant Art”