The word "although" is similar in meaning to "but". There are a few differences in how they're used, though. "But" is used to connect ideas that conflict with each other, but these ideas are somehow related to each other:

She's moving to London, but she'll be back to visit.

When you use "although" in spoken English, it usually connects two conflicting ideas. But these ideas are each a completely separate thought. When you start a new sentence with "although", it sounds like you weren't planning on saying this thought when you finished your last sentence:

She's moving to London so we probably won't see her much any more. Although I'm sure she'll be back to visit.