What is the difference between /usr/local/lib and /usr/lib inside under Linux?

  linux, python

The environment is Ubuntu14.04. When using the command which python3, you get a link:

/usr/local/bin/python3

Point to:

/usr/local/lib/python3.4

However, under /usr/lib/

/usr/lib/python3.4

May I ask why there are two such folders? Which is effective, or both?

Thank you.

There is a small story that I don’t know if the subject knows, usr does not mean user, but unix system resrouces, which is helpful to understand the sub-file system /usr.
/usr/bin:Almost all system executables will be installed here.
/usr/local/bin:It is possible to store some system users’ own specific executable files without worrying about being overwritten or destroyed by actions such as system upgrade. This directory is not necessary

Hope it helps ~