An ELT Glossary : Adverb

"Adverb" is a difficult word class to define, as it contains a number of items which differ from each other in various ways. As Quirk et al (1972:267) say"...it is tempting to say simply that the adverb is an item that does not fit the definitions for other parts of speech."

Richards, Platt & Weber (1985:6) describe it as "a word that describes or adds to the meaning of a verb, an adjective, another adverb or a sentence, and which answers such questions as how? where? or when?"  Some examples :
Modifying a verb : He spoke slowly. 
Modifying an adjective : I was really tired,
Modifying another adverb : He spoke really slowly.
Modifying a sentence : Frankly, I expected him to fail.

There are therefore many different types of adverb,  usually described with reference to their meaning. To give just a few examples:
Adverbs of manner (many of which end in -ly, some in - wise, others which have no morphological indicator) : slowly, clockwise, fast etc
Adverbs of place : here, there, locally etc
Adverbs of frequency : often, sometimes, never etc
Adverbs of time : today, then, now etc
Intensifying adverbs : really, quite, incredibly etc
Sentence adverbs, Frankly, Hopedfully, Interestingly, Luckily, etc
Connectives : However, Consequently, Moreover, Secondly etc

However, adverbs also divide into syntactic groups and may be adjuncts, disjuncts or conjuncts.

See also adverbial,   adverb particle,   adverbial clause.


References

Quirk, R. et al (1972) A Grammar of Contemporary English, Longman
Richards, J., Platt, J., Weber, H. (1985), Longman Dictionary of Applied Linguistics, Longman