These aren't all of them, but I think they are some of the most useful.

# making a directory in another directory that doesn't yet exist...
FileUtils.mkdir_p '/path/to/your/directory/that/doesnt/exist/yet'
recursively remove a directory and the contents
FileUtils.rm_rf("/path/to/directory/you/want/to/delete")
write a file from the contents of another file...
File.open("/path/to/the/file.ext", "wb") {|f| f.write(@your_other_file.read)}