1. (Not important) 'However' is more formal than 'although'
2. I can use 'however' in an independent sentence - I can't use 'although' in the same way.
"However, I don't see any way to do it." You can't start the same sentence with 'although'.
3. As part of a dependent clause, 'however' cannot start the sentence, while 'although' can.
"He is not talkative, however, he is actually quite friendly."
"Although he is not talkative, he is actually quite friendly."