TVDB is used by Plex and other services as their official database for TV shows. If you put tv seasons and episodes in the same order as TVDB, the episode titles and descriptions will line up correctly. Sometimes a downloaded tv episode is named incorrectly causing problems. It's a good idead to check the TVDB before you import episodes into a Plex library.
You can also check TVDB for movies though this is a secondary consideration.
TMDB is more of a searching and tracking service. Reviewing titles is free. You can register if you want to track what titles you have and what you want. It supplies more detailed information than TVDB. In that regard, it's very similar to imdb.
IMDb is an online database of information related to films, television programs, home videos, video games, and streaming content online – including cast, production crew and personal biographies, plot summaries, trivia, ratings, and fan and critical reviews. IMDB is owned by Amazon.