STATICFILES_DIRS
: You can keep the static files for your project here e.g. the ones used by your templates.
STATIC_ROOT
: leave this empty, when you do manage.py collectstatic
, it will search for all the static files on your system and move them here. Your static file server is supposed to be mapped to this folder wherever it is located. Check it after running collectstatic and you'll find the directory structure django has built.
--------Edit----------------
As pointed out by @DarkCygnus, STATIC_ROOT should point at a directory on your filesystem, the folder should be empty since it will be populated by Django.
STATIC_ROOT = os.path.join(BASE_DIR, 'staticfiles')
or
STATIC_ROOT = '/opt/web/project/static_files'
--------End Edit -----------------
STATIC_URL
: '/static/' is usually fine, it's just a prefix for static files.