It automatically creates a set features that you can then use a final layer of machine learning to get what you want.

In machine learning, normally you have to create a set of features (call feature engineering - basically think algorithms to better represent your data). The amazing thing about deep learning is that the computer does this for you!

You just need a few 10s/100s face/nonface images - same for 20,000 other objects - this is called fine-tuning.

For more, andrew ng, geoff hinton, yann lecun have given talks on this at google and they are up on youtube.