The difference between Kontakt (sometimes called “Full Kontakt”) and the free Kontakt Player can be confusing, so it’s smart of you to ask this!
Kontakt (often called “full Kontakt,” or “the full version of Kontakt”) is a full fledged sampler that you can create your instruments with, and more importantly, you can load instruments that other people created with Kontakt. Many Kontakt libraries require the “full version” of Kontakt, which costs $399 (MSRP).
Kontakt Player is a free version of Kontakt. You can’t create your own instruments with it, but you can load instruments that are labelled as a “Kontakt Player Library.” To become a Kontakt Player library, we (developers like Realitone) pay a fee to Native Instruments, who then sprinkle magic fairy dust on our library, so that it will work in the free Kontakt Player.
For our entire catalogue, we have paid this fee so that they will work in the free Kontakt Player. It adds to our costs, of course, but it’s worth it because this fee is proportionately not very significant compared to the cost of the library.
For some of our legacy products like Realibanjo (legacy) or RealiWhistle (legacy), the full version of kontakt is required. However, if you are an owner of a legacy product, there is an upgrade path available so that you can get the newer "Kontakt Player Library" version. See HERE for details on that.
If you’ve never played with Kontakt at all, you can download the free Kontakt Player here:
https://www.native-instruments.com/en/products/komplete/samplers/kontakt-6-player/
The download is free and they even include some free instruments, so you can get a feel for it. If you decide to purchase the full version of Kontakt, then your Kontakt Player simply becomes "Kontakt".
But wait!!! If you buy a qualifying Kontakt Player library (like Realivox,) then they give you a pretty deep “Crossgrade” discount!The funny thing about that, is that if someone wanted to buy Kontakt, they might as well buy Realivox Blue first, because they’ll save more money than what they spent on Realivox Blue! In other words, it’s like getting Blue for free. ($149 + $249 crossgrade price is still less than $399 - the price of kontakt by itself)