[attheodo]

e·van·ge·list: A person who seeks to convert others to the Christian faith, esp. by preaching.

Where "Christian" you can put whichever company grows along with it's API/code/lib usage and utilization by others.

As I see it it's a purely "hype" role and it's essentially a tech-aware marketeer seeking to attract fellow developers and engineers to work and experiment with his company's platform and products.

[ecspike]

It depends on the role. It can range from a tech-aware marketeer to an actual software engineer by trade who writes code samples and materials for the company's website or some combination there of.

[metajack]

It may be a role focused on hype, but many of the people who hold these roles are developers themselves. I have no idea if Adria codes, but many evangelists are more than just marketers. This is true of any job–you never know who is a programmer–but developer relations people more so than many.