On OS X El Capitan, what is the difference between /Library/ and /System/Library/? There are two 'Library' directories in my OS X 10.11 installation, one being /Library/, the other being /System/Library/.
What is the difference between different Library folders?
/System/Library is for stuff that doesn't change and is installed by Apple. Files here help the operating system run.
/Library is for things that may change and that pertain to all users on a computer.
And for completeness (although not mentioned in the question):
~/Library is where things that may change specific to a particular user go.