better with examples : I like coffee but without sugar. I took the way to go to Seoul whereas you took the way to go to Busan

Both "but" and "whereas" introduce a contrast between two sentences. In the case of "but" the contrast is usually stronger, more opposing: "I don't like sport in general, but I actually enjoy playing soccer" however, "whereas" is used to compare two opposing sentences: "This burger is tasty and juicy, whereas this other one is slightly overcooked"

"But" is like the French word "mais." "Whereas" is much less common. It is more formal. Whereas is used to compare two things. Example: I go to school at 8 o'clock, whereas my friend goes to school at 8:30. Whereas is showing there is a difference between the two times. It barely ever used in common speech. Example: I want to go to the movies, but my dad said no. You could not use whereas because nothing is being compared. I hope that helps!