On E2, virtually every page is a node and many of the items contained in a page are nodes. That's both technical and vague so here's how it works for the average user: In its most commonly used context, a "node" is a collection of one or more writeups/articles, such as the page you are viewing now. Don't let the title mislead you. This here is a node that happens to bear the title "node" because it talks about nodes. Other systems may call it a "topic" and you should think of it as such if it helps.
If you are just viewing my writeup, without the others below, then you are on the single writeup display page (which, rather confusingly, is a node in its own right) and should click the link all of node, there are X more writeups in this node that you see just above this writeup. You can also adjust how you view E2 by going to your preferences link in your Epicenter nodelet on the right. We highly recommend you choose display type "full" because most people use it and all writeups are displayed.
What you're really viewing when you're looking at a node like this one is a type of node called an "E2node". This is what's generally meant when someone says "node." A single e2node may contain any number of writeups by any number of users. A writeup is what you are reading right now, this is "Virgil's" writeup for the node "node". There are a number of writeups below mine that give different definitions for different contexts of the word "node". If there were a song called "node" its lyrics would most likely be below. If there were a play entitled "node" the play's author and basic plot would most likely be below. You may also come across a node that contains no writeups, in which case we call it a "nodeshell." A nodeshell is still a valid e2node, just one with no content. If you create an e2node, it will be a nodeshell until you post your writeup.
Users will often refer to writeups as nodes or nodes as writeups. This is technically incorrect but you can't fight common usage. For instance, a user would say in the chatterbox "I'm working on a new node" or "Check out my node on reptiles". What the user really means is "Check out my writeup in reptiles". Nobody "owns" a node. It makes little difference if your writeup is first or fifth. After fifth you might want to consider not adding a writeup unless you've really got something new to say.
Besides "E2nodes" there is a "document" node type. A 'document' node is one large page with a single "writeup". A document will not have a particular author assigned to it - only E2 staff can create a document. For examples of different documents check out