The term "piano player" is often used to refer to someone who plays the piano as a hobby or for entertainment, while the term "pianist" is more commonly used to refer to someone who plays the piano professionally or as a serious musician.
Pianists typically have a higher level of musical training and skill than piano players, and they are often able to play a wider variety of music. Piano players may simply be able to play a few songs or melodies by ear, while pianists are often able to read and perform complex musical compositions.
In addition, pianists typically have a greater understanding of music theory and harmony, and they are able to use this knowledge to create their own music and improvise. Piano players, on the other hand, may be more focused on playing the music that they know and enjoy, rather than on creating their own music.