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Each word in a sentence belongs to one or another class of grammar. These classes are collectively called "the parts of speech". Here's an old mnemonic children's teaching rhyme (possibly Edwardian), describing the eight parts of speech in English grammar:

"Every name is called a NOUN,
  As field and fountain, street and town;
In place of noun the PRONOUN stands,
  As he and she can clap their hands;
The ADJECTIVE describes a thing,
  As magic wand and bridal ring;
The VERB means action, something done -
  To read and write, to jump and run;
How things are done, the ADVERBS tell,
  As quickly, slowly, badly, well;
The PREPOSITION shows relation,
  As in the street, or at the station;
CONJUNCTIONS join, in many ways,
  Sentences, words, or phrase and phrase;
The INTERJECTION cries out, 'Hark!
  I need an exclamation mark!'
Through Poetry, we learn how each
  of these make up the PARTS OF SPEECH."

Modern language tuition has since substituted the DETERMINER (an indefinite/definite article such as a, an or the) for the INTERJECTION as the eighth part of speech. Peter Hobbs therefore suggests this humourous addition to the poem:

"But oh! The modern now prefer
  The relative DETERMINER...'

A separate page exists detailing mnemonics for conjunctions.