For the 4th photo, I think you should crop the image a bit to correct the angle of the building and flagpoles. They seem to be leaning towards the right, and it looks a little weird to me.
I like the photo of the dragon statue a lot, but I think it may be a bit over exposed. It looks kind of washed out in the background.
To attempt to answer your question about exposure, the way I understand it is this. An overexposed photo, is one where the light areas are blown out, and pure white, (or just overly bright, which can be wrong for some scenes) resulting in loss of detail. For example, the sky in your last photo. (Of course, if you had exposed to get the details in the sky, the rest of your photo may have been to dark to make out).
Underexposure is pretty much the same thing, just affecting the shadows of an image.
Hope that helps, but someone with more experience could probably provide a better explanation.
Also congrats on making it to Beijing. That's pretty awesome.