Normally nouns are concrete: "One book is, two books are..."
Some nouns are abstract: "The electorate is unhappy with Bush"; "The public is pleased with Obama." These are called abstract because they employ an act of cognition to separate relevant individuals from the masses and to assimilate them into one entity to be represented by a single word.
Sometimes these are used as plurals, especially in the UK: "The public are not happy with Gordon Brown."
Uncountable or uncount nouns are used in the opposite manner, as collective singulars: "My hair is messy." "The money is ready."