hi,
just to add on to what srinivasans explained: the 4 p's of marketing (viz. product,price,place,promotion) are collectively known as the marketing mix and Promotion or sales mgt as it is otherwise known, consists of personal selling, sales promotion, advertising, publicity and
PR(collectively known as the promotion mix). hence, advertising is a part of the promotion mix which in turn is a part of the marketing mix. Hope this helps..