There are 2 types of modders that i am aware of (if we analyzed this more there are probably a wide variety of modders). All of us fall into one, or the other category.. Perhaps both.
1) There is the type of modder i like to call a "Code Monkey". Skilled at C# or whatever the programming language of the week it is. All they do is mess around with the games code, or Like Harpo, Writes Tools for the people that don't know shit about code so they can mod the game.
2) The other type is "The Artist". They really know nothing about game code itself, but they do know their way around Modeling Programs such as XSI, or 3DSMax, and Image Editors like Photoshop.
There is probably a third category which is the "Tinkerer". The modder is just learning how to do things, but doesn't know enough to actually make a mod. He/she is learning what makes things tick, or is just experimenting with models, and textures.
I more than likely fall into the "artist" category since i know absolutely nothing about programming, but I can Model, and Texture. I can change some gameplay aspects if it is simple stuff that can be done in wordpad, but can not "create" new stuff using a programming language.
All modders know enough that they probably could be employed in the gaming industry, but just not enough to actually get hired.