Dan is a good dancer.
(good is an adjective describing the noun dancer)
Dan dances well.
(well is an adverb describing the verb dances)
Adverbs can describe verbs, adjectives, other adverbs, or entire clauses.
While most adverbs end in -ly, not all do. Some adverbs don't follow a pattern when they're formed with adjectives.
Dan is a good dancer.
Dan dances well.
Here are some adverbs that end in -ly:
Here are some examples of irregular adverbs: