An adverb is a word or phrase that modifies or qualifies an adjective, verb, or other adverb or a phrase, expressing a relation of place, time, etc. There are various kinds of adverbs such as adverbs of degree, manner, time, frequency, etc. Instances of the same are slowly, cutely, nicely, finally, etc.
Let us take an example to study the difference between the two:
The word "Hard" can be used as both an adjective and adverb. As you know, an adjective is a word that adds to, describes or qualifies a noun whereas an adverb adds to or describes a verb.
Let us look at an example:
Hard as an adverb:
Michael threw the football hard .
(Here, "Hard" describes the verb threw and answers the question "how?")