The implementation varies somewhat between Linux distributions, but all seek to implement the Linux Filesystem Hierarchy Standard for the most part. Some distributions more so than others. The latest version is 2.3 and can be viewed via the links in the URL above or directly via FHS 2.3 PDF
Take time to view the document and compare it with the implementation Ubuntu uses. It will provide you with a good background to where and why files are located where they are in Linux, and also give you a quick lesson on how this is more of a goal for distributions than a hard requirement, as distributions are pretty much free to put things anywhere they want.
