Adverb

Definition:

The part of speech (or word class) that is primarily used to modify a verb, adjective, or other adverb. Adverbs can also modify prepositional phrases, subordinate clauses, and complete sentences. Adjective: adverbial.

Positions of an Adverb:
An adverb that modifies an adjective ("quite sad") or another adverb ("very carelessly") appears immediately in front of the word it modifies. An adverb that modifies a verb is generally more flexible: it may appear before or after the verb it modifies ("softly sang" or "sang softly"), or it may appear at the beginning of the sentence ("Softly she sang to the baby"). The position of the adverb may have an effect on the meaning of the sentence.
When used with a verb, adverbs can give information about:

• how something happens or is done:

She stretched lazily.
He walked slowly.
The town is easily accessible by road.

• where something happens:

I live here.
She’s travelling abroad.
The children tiptoed upstairs.

• when something happens:

They visited us yesterday.
I have to leave soon.
He still lives in London.

Adverbs can make the meaning of a verb, adjective, or other adverb stronger or weaker:

• with a verb:

I almost fell asleep.
He really means it.

• with an adjective:

These schemes are very clever.
This is a slightly better result

• with another adverb:

They nearly always get home late.
The answer to both questions is really rather simple.
Adverbs are often found between the subject and its verb:
She carefully avoided my eye.

They can also come between an auxiliary verb (such as be or have) and a main verb:
The concert was suddenly cancelled.
He had quickly eaten his dinner. 

Functions of an Adverb:
Adverbs typically add information about time (rarely, frequently, tomorrow), manner (slowly, quickly, willingly), or place (here, there, everywhere) in addition to a wide range of other meanings.

http://www.oxforddictionaries.com/words/adverbs
http://grammar.about.com/od/ab/g/adverbterm.htm