Hi shramana,
In simple terms, both the words "but" and "although" can be referred to as 'contrast' keywords. If you're thinking in terms of which word to use in a specific SC prompt, then you should post the entire prompt (including the 5 answer choices), since there are likely additional clues in the sentence that would help to define which word would be correct in the context of the sentence. If you're working through a particular practice questions, then it's also possible that that question has been discussed in the SC sub-forum on GMATClub - so you might want to try "Googling" it and reading through the explanation/discussion of the prompt.
GMAT assassins aren't born, they're made,
Rich