Blurring Lines: Affiliate vs. Agency

Just what is the difference between an affiliate and an agency?

One of the interesting discussion points I heard recently was the 'competition' between affiliate programs and ad agencies.

Let's see:
1. They both work on behalf of clients.
2. They both should work on raising prospect interest and converting it into action.
3. They both should be paid for performance.

Now some of the tools and channels are different, but that distinction will dissolve over time. So, as usual an early categorization scheme isn't very useful over the long term as the industry changes. It is likely that the emphasis on measurability and tracking demanded of affiliates will bleed into the agency world.