Impostor syndrome is that crappy feeling you get when you convince yourself that you are stupid and have no idea how you got where you are. You feel like a big fake that will soon get discovered for being a big fake. It's a feeling of total defeat. It sucks. That said, it can be a good thing. I recently saw a Network Chuck video on this very subject. It's titled "I'm feeling stupid today."
Chuck says that this feeling is normal, and can actually be a good thing. You can't let it defeat you.
If you're not feeling stupid, there's a good chance that you're not challenging yourself. He also touches a bit on the Dunning-Kruger effect, which is basically a feeling where someone who sucks overestimates their own ability. It's related to something called illusory superiority, which I haven't read as much about, but simply put, it just means someone who sucks at something thinks they are better at it than they are because they actually don't have the cognitive ability to recognize that they come up short in their abilities -- or something like that.
Anyway, people with a bit more knowledge become increasingly aware of what they don't know and of what their shortcomings are. This kind of leads to you feeling like a big poop when it comes to skills.
The truth is, you should feel a bit clueless about what you're doing. If you get a little bit of that impostor syndrome feeling, you are probably in the right place. You're being challenged. You're being put in a position where you are often required to work outside of your comfort zone. This is amazingly helpful in helping you build your skills and helping you learn to be better at what you do.
Of course the trick is to not let it take over and break you. Just run with it. You'll do alright!
Here's another Network Chuck video on the very same subject: