Neuro-Linguistic Programming (NLP) is, at a high level, a study of learning, language, and psychology.  There are two specific branches to the field.  One branch studies how human beings learn skills.  The other branch studies how the use of language affects us.

Those who study the skill-learning branch of NLP focus on using the techniques of NLP to gain or improve their skills.  At a very high level, NLP tells you to find someone who has a skill you would like to have, or is an expert in that skill.  You examine how that person performs the activities associated with the skill to identify specific mental processes and actions they go through.  Then you "model" those behaviors in yourself to learn or improve the skill.  In theory, you'll eventually become as much an expert with the skill as the person you originally studied.  You should also have a pretty good idea how to teach others this same skill.

Those who study the language side of NLP look at how the use of words, phrases, and imagery can have a profound effect on the audience hearing or reading those words.  Not only can this knowledge of NLP make you a more effective communicator, it can also (it is said) help you to more effectively persuade and influence your audience.  I've read (some paranoid) articles claiming that it's possible to word a document such that on the surface it praises someone highly, while (through the use of NLP) leaves the subconscious impression on the reader that the subject of the document isn't deserving of the praise and is in fact incompetent, stupid, crooked, etc.

I'm interested in both sides of the NLP "coin" but especially the use of language, as I am a writer by nature.  More on NLP as I learn it (through a few books I've picked up at Amazon).