I know a little of dom, and would like to learn about ElementTree. Python 2.6 has a somewhat older implementation of ElementTree, but still usable. However, it looks like it
It is the same library (same API, same features) but ElementTree is implemented in Python and cElementTree is implemented in C.
If you can, use the C implementation because it is optimized for fast parsing and low memory use, and is 15-20 times faster than the Python implementation.
Use the Python version if you are in a limited environment (C library loading not allowed).