Indefinite articles, a and an, are not used with plural nouns or uncountable nouns. They are used

• when a singular countable noun is talked about for the first time,
• when classifying, describing and generalizing,
• in exclamations following words such as 'what' and 'such',
• in expressions like: 'a kind of', 'a sort of' and 'a type of'.

http://www.bbc.co.uk/learningenglish/english/course/upper-intermediate/unit-10/session-1