The term "Agile" came into existence in February 2001 when the "lightweight method leaders" at the time met to agree on common principles. Each of their approaches had key aspects in common. This was an opportunity to elevate the discussion.
Since that time, Agile has been dominated by Scrum with a bit of Extreme Programming and a growing amount of Lean. Each details practices which help you to achieve the principles. Scrum starts from a management point of view. It answers what your process should look like in a prescriptive fashion. This makes it easy to follow. But it doesn't go much into engineering practices. Extreme Programming offers lots of insights in that area (see this previous blog entry for more on this). Lean also adds some useful principles and practices (it is not a subset, but rather has significant overlap). The three complement each other very well. Thus I always tend to say that I do Agile rather than Scrum or XP or Lean as I think it is best to combine them.
One problem that occurs is how do you determine if you're Agile? It is easy to say that you're Agile, but harder to achieve its benefits. So if it is not clear what is Agile, then Agile's success is blurred because many of its "practitioners" aren't really doing it.
As described above, Agile is a combination of principles and practices. People tend to learn Agile via the practices. These outline common ways people have found to achieve the principles. But, I would argue that the principles define whether you're Agile or not. If your organization doesn't believe in or follow one of the principles, you're probably not Agile. Similarly, if you don't follow a specific Agile practice, but you adhere to the principles (i.e., you've found another way to achieve them), then I would argue that you are Agile.