Can someone explain something to me (like I'm five)?

I see discussion about how people will be able to make transaction fees. This seems to appear in discussions about why miners would still be interested in the coin once mining becomes too difficult or all the coins are distributed (according to the possible new schedule Evan talked about). Also seems to come up in discussions about "nodes." What are nodes? Are they part of the Darksend implementation?

Sorry if I'm confusing a number of different issues, just trying to understand a little better.