Adverbs are used to describe or modify a verb, adjective, clause, or another adverb. Basically, they modify everything except nouns and pronouns (which are modified by adjectives).
Example of an adverb modifying a verb:
He was running fast. (fast modifies running)
Example of an adverb modifying an adjective:
She took a very small piece of the cake. (very modifies small)
Example of an adverb modifying a sentence:
Strangely, the man left the room. (strangely modifies the whole sentence)
Usually adverbs answer to the questions “When?” (adverbs of time), “Where?” (adverbs of place), and “How?” (adverbs of manner).
Adverbs can also be used to connect clauses and sentences (in this case they are called conjunctive adverbs). For example:
It was dark. Therefore, we needed the torch. (therefore connects the two sentences)