PCA basically is a projection of a higher-dimensional space into a lower dimensional space while preserving as much information as possible.
I wrote a blog post where I explain PCA via the projection of a 3D-teapot:...
...onto a 2D-plane while preserving as much information as possible:
Details and full R-code can be found in the post:
http://blog.ephorie.de/intuition-for-principal-component-analysis-pca