Sample Libraries often require software called a "sampler" to run. In most cases, this is Native Instruments' program Kontakt. Some of our libraries do require Kontakt, whereas others do not.
On each product page you will find a small Product Info box on the left hand side of the page. Listed here will be 1 of 3 possible messages:
If you see this message, you will be able to use the free version of Kontakt with this library, named Kontakt Player. You can download Kontakt Player here.
This message means that you'd require the full and paid version of Kontakt. Using the free Kontakt Player with a Kontakt Full library will leave the product in 'DEMO' mode. You can purchase the full version of Kontakt here.
The above message means that the library does not use Kontakt at all, and will instead load as a standalone 'Instrument' in your DAW. Examples of this are Hans Zimmer Strings, Eric Whitacre Choir, and BT Phobos.