The distinction is important, because a host of English words can operate as several different parts of speech, depending on the circumstances of their use. Take for example two very common words, table and fast. Most people would say that 'table' is a noun and that 'fast' is an adjective; they would be quite right, as these simple sentences illustrate:
A Part of Speech does not define a word exhaustively; it merely describes its current function.
|1 The table is brown, rectangular and polished.|
|2 He caught a fast train to London.|
However, table can also be used as an adjective or as a verb:
She bought a beautifully carved table lamp.
Here table, like 'beautifully carved', describes the lamp
The MP for Derby tabled a motion in the Commons.
Here tabled denotes an action, something done.
Similarly, fast can have other functions - an adverb:
She ran fast
Here fast modifies (gives more information about) the verb 'ran', telling us how she performed the action.