The word decision is an abstract noun.
It is abstract because it cannot be touched or felt - it just exists, the same way love or hatred (which are also abstract nouns) exist.
Collective nouns are nouns such as council, jury - where one word is used to denote many parts which are included in it. Proper nouns are names. Compound nouns are made up of two or more words, such as brother-in-law, etc.