An adverb modifies a verb, an adjective, or another adverb. In this case, "modifies" means "tells more about." An adverb tells more about how the verb is being done. Many adverbs end in "-ly."
Susan writes quickly and well.
Herbie will visit tomorrow.
Let's go home.
That was a very funny joke.
Adverbs can answer questions like these: "How?" (quickly and well) "When?" (tomorrow) "Where?" (home) "To what extent?" (very funny)
An interrogative adverb asks a question. The interrogative adverbs are how, when, where, and why.
How did you get here?
Where are you going next?
A conjunctive adverb joins two ideas. It can give emphasis to one of the ideas, or answer the question "How are they related?" Some common conjunctive adverbs are besides, however, indeed, moreover, nevertheless, otherwise, and therefore.
I am allergic to cats; nevertheless, I love them.
It might rain later; therefore, we should pack our umbrellas.
A semicolon is used before a conjunctive adverb, and a comma is used after it.
See also: Adverbs and Adjectives Versus Adverbs.