What is a node type?

My boss has given us the assignment of learning about nodes and node types (sort of a self training exercise, I guess). I don't want anyone here to give me the definition, but I would be really grateful if someone could point me in the direction of websites with explanations and information. I've found a few, but they are all pretty advanced, and I'm looking for the more simplistic definitions before moving onto the advanced.

TIA

 

You didn't mention the scale of your target project, but here is a good link

http://www.walruscomputers.com/tomes/Tomes_Volume_Six-Networking_Tutorial.htm