PlexKodiConnect (sometimes seen abbreviated as PKC) is an add-on which adds your Plex content in a regular Kodi library. Thus you get the normal Kodi experience but with Plex as the backend source running hidden. This add-on is not found in Kodi’s repository as, from what I understand, it makes some lower level changes to Kodi that the Kodi devs are not supportive of. If you are interested in trying it I would suggest to do some reading first and make a backup of your Kodi settings.
The official Plex add-on (the one under discussion in this thread and is found in Kodi’s repo) is very similar to the Plex app you would find on other platforms, just a bit outdated at the moment (ie it doesn’t have some of the newer features like the new streaming service stuff).
There is also a “Plex for Kodi” beta that can be found on Plex’s forum which has a few versions but all of which are spun off from the official Plex add-on.
Composite is another Plex client that can be found in Kodi’s repo and it more of a basic directory tree type interface. It seems to function quite well from my limited testing.