An adverb can describes a verb by modifying or qualifying it.  For example, see the following sentence:

He yelled loudly.

In this sentence the verb, yelled, is modified by the adverb, loudly.  How did he yell?  He yelled loudly.

Here are some other examples:

She drove slowly.

The teacher spoke accordingly.

The dog ran quickly.

In addition to modifying nouns, adverbs can also modify adjectives or other adverbs.

For example:

She is really beautiful. (Adjective: beautiful.  Adverb: really)

Her dog runs extremely slowly. (Adverb: slowly. Adverb: extremely)

Adverb Forms

There are several forms of adverbs.  Here are a few:

Adverbs of purpose:

He ran up the hill quickly to beat his opponent.

She looked under several tables to see if she could find her keys.

Adverbs of Frequency:

The dog eats a bone every day.

He often runs out of time.

Adverbs of Manner:

He moved quickly to beat his opponent.

She performed her job accordingly.

Adverbs of Place

She laid on her bed for several days.

She stays there now.

Other Forms of Adverbs

An adverbial phrase is formed when a group of words does not contain the verb or subject but acts as an adverb (this often occurs in prepositional phrases):

She went to the store.

He spends his day on a tractor.

An adverb clause is formed when a group of words contains both a verb and a subject, but also work as an adverb:

When she comes back from the store, we're going out to eat.

When he gets off the tractor, let's go greet him.

An additional form of adverb occurs in an infinitive phrase:

They ran to the store to buy some apples.

The dog crawled under the fence to fetch his bone.