There are two versions of Kontakt: The free Kontakt Player and the full version (just "Kontakt" or Kontakt Full).
The Full Version can load and play all collections without limitations. The Player can only load collections licensed for it, meaning collections where Orchestral Tools has paid Native Instruments to make these collections compatible with the free version.
Player collections need to be registered inside Kontakt before first use. They also have an entry in Kontakt's Libraries tab, whereas non-Player collections do not require registration, but have no entry in the Libraries tab and need to be loaded from the Files tab.
There are no other sonic or functional differences as far as our collections are concerned and a Player collection will work just fine in the Full Version. On the other hand, a collection requiring the full version will only produce sound for 15 minutes when loaded into Kontakt Player.
The Current Versions section lists the minimum version of Kontakt for each collection as well as whether the full version is required or just the Kontakt Player. Of course any Player collection also works fine in the full version.
Unfortunately getting a Player license is rather expensive and does only make sense for rather large collections. We have also found that most of our customers have a Full Version license. There are a lot of awesome products from different vendors that require the full version, so it makes sense to get it sooner or later anyway.
If you try to load a non-Player patch into Kontakt Player it will load fine, but will be in Demo Mode. In Demo Mode, sound will fade out after 15 minutes of use and you will only be able to use the patch again after reloading it.
If you already own one or more Kontakt Player-enabled collections from any vendor you can purchase a license for the Full Version of Kontakt at a discount. The Native Instruments website has more info.
If you have the full version of Kontakt, all Player collections will look and work in the same way as in the Player. There is no difference.
Non-Player collections will run without demo restrictions, but they do not get the advantages of Player collections. They can not be added to the library tab and do not need to be registered.