WN is short for Web Novel while LN is short for Light Novel.
Usually a novel starts out as a web novel, a WN. This is because they can simply be published on the web, so there isn't really much of a cost associated with it. If it has enough popularity, then a publishing company would naturally want to publish it as a light novel in order to make money.
They don't just take the web novel, print it out and then that's it. When a novel goes from WN to LN it is edited, sometimes a lot, and artwork is added. For instance, when Overlord went from WN to LN another character was added: Albedo. In other instances, the editing process is more subtle though and not always that extreme.
Now, I also just mentioned artwork, so let's cover that a bit more to cover all bases. After Overlord was published as a light novel it was also published as a manga. This is a very common process in Japan that a light novel is also published as a manga. On the other hand it's more rare that a manga will be published as a light novel.
So in short?
Web novel is like a beta version, while the light novel is the finished released version. The manga is there for the pretty pictures!