Agencies traditionally were either above the line (atl) or below-the-line (btl).
Above originally being commission based - TV, radio, posters, press, etc.
Below-the-line print based - direct mail, point of purchase, leaflets, etc.
However this is 50 years out of date and the more accurate definition is by marketing definitions.

All agencies are concerned with building brands but for an atl agency it's their main focus. Their advertising (Brand Marketing) seeks to change minds and create a unique image for the brand. Traditionally they use TV, posters and press to do this. However, with the movement of more clients budgets to below-the-line they are having to be more flexible.

Below-the-line agencies practice Direct Marketing and Promotional Marketing. Because brands are important almost all of their activity is also building the brand. Many clients are now putting their money below-the-line as it is closer to the sale and is more accountable. This has forced btl agencies to be more ttl and many are now doing the client's atl work as well.

Direct Marketing isn't just about big telephone numbers or direct mail, it also uses TV, press, telephone, internet and more recently, posters. It's actually far more complicated than most people think with many different disciplines. The key factor about direct is that it seeks to talk directly with you (hence the direct) and aims to set up a dialogue. Whereas above-the-line advertising seeks to tell you about the brand and change your view of it, direct seeks to build a relationship between you and the brand. and get you to change your behaviour. In simple terms, atl talks at you, btl talks to you.

Promotional marketing uses all the media available as well as point of purchase and on-pack. It seems to level sales though incentives like competitions or offers. Go in to any supermarket and you'll find a large percentage of goods have some promotion.
Below-the-line agencies are also doing more Field Marketing - using people (out in the field) to sell direct, sample, demonstrate products - the Marlborough girls are famous. Closely linked is Event Marketing - many of the drink companies have abandoned TV for this as it allows people to have a greater experience of a brand - some people now call this Experiential Marketing. Sponsorship is another important area as this associates brands with powerful events like the Olympics.

As more and more of client money is going into direct, promotional, field, event, experiential, on-line and sponsorship it's forcing the industry to expand at a vigorous rate (atl has been shrinking for many years) so job opportunities, salaries, job security and promotions are much better.