The first option will install the library. The second option will only build it (I'm guessing the library includes a C extension), but will not install it. You would use the second option if you have an unsual setup and want to install the package manually. 99.99% of the time, you want the first option (to install). site-packages refers to the location (on your computer) where Python looks for 3rd party libraries, e.g. on Windows it would be something like C:\Python27\Lib\site-packages. The site-packages subdirectly is used to distinguish 3rd party libraries form the standard modules. The word "site" in this case refers to the site on which Python is deployed (i.e. your computer).