The call to the use-package macro will load a package either
immediately, or when the package is first used (via autoloading). In the
simplest case, a use-package declaration loads a package when
it is evaluated.1 If the declaration is in your
init file, this happens automatically each time Emacs is started.
For example, the declaration below immediately loads the library
foo, just like require would:
(use-package foo)
If the library ‘foo’ is not available in your load-path,
the declaration logs a warning to the ‘*Messages*’ buffer.
Note that a “package” is different from an Emacs Lisp “library”.
The above declaration tells use-package to load the library
foo.el, which in the overwhelming majority of cases also
resides in a package named foo. But the package
foo might also contain a library named foo-extra.el. If
that library is not loaded automatically, you will need a separate
use-package declaration to make sure that it is loaded when
needed. This manual will often use the terms “package” and
“library” interchangeably, as this distinction does not usually
matter, but you should keep it in mind for the cases when it does.
The details of how and when you should load a package might differ from one package to another. When in doubt, refer to the package documentation for details.