They may take on various forms:

Water is important for survival.	(Single words)
Mr. Jones spoke to Dr. James.	(Proper names)
The boy ate an apple.	(Nouns and articles)
My friend works with her father.	(Nouns and possessives)
The young girl wore a long, white dress.	(Nouns and adjectives)
Some of the kids ate all of the cake.	(Nouns and quantifiers)
The man with the gun frightened the people in the bank.	(Nouns and prep. phrases)
The woman who lives there is my aunt.	(Nouns and relative clauses)
The dogs sleeping on the deck should be left alone.	(Nouns and phrases)
Whoever wrote this is in trouble.	(Noun clauses)

Pronouns and similar words can also function as subjects and objects:

He gave the money tous.
Someone leftthis.
  Giveme one of each.