What's a Node?
Often called "pools" or "schools", nodes are areas of open water where a specific type of fish are guaranteed to be caught. When you cast into a node, regardless your skill or equipment, you will catch fish of the type identified by the node. So when you cast into a Sagefish School, for example, you can plan to catch Raw Sagefish.
Each node lasts for as long as there are things to catch. Usually that means 3-5 catches, but it can be more or less. You will know when you run out of things to catch since the node will disappear.
Nodes don't always contain fish. Some, like Floating Wreckage contain other items, like a Mithril Bound Trunk. Boxes and trunks will contain other items, one in particualr being a Weather-Beaten Journal. More about that on my Items of Interest page.
Reasons to Fish in Nodes
The best reason to focus on fishing nodes is that, if you are looking for a specific fish and there are nodes that contain them it will be by far your most productive way to catch what you are looking for.
Also, nodes have no skill-level requirement. At level 1 fishing you can catch Emperor Salmon in an Emperor Salmon School.
So, why train Fishing if Nodes are so great?
There are some fish that simply can't be fished from pools. One example is Old Crafty, a fun cross-faction catch for Alliance characters. The horde equivalent is Old Ironjaw, which is a lot harder in my opinion. Finally, you were going to be an Angler and get that Salty title anyway, right?