Difference between OpenCV Haar Cascading Classifiers

Question

OpenCV provides several cool Haar Cascade face detection classes:

- haarcascade_frontalface_alt.xml 
- haarcascade_frontalface_alt2.xml
- haarcascade_frontalface_alt_tree.xml
- haarcascade_frontalface_default.xml

I wonder what the difference is between these classifiers.

I noticed that in the application the "default" version returns more false positives than the "alt" and "alt2" versions; and the version of "alt_tree" is the strictest. However, I am looking for their fundamental differences. For example, if someone knows which database is used to train them.

thank