Please consider the following two sentences:
Although these products are expensive, retailers have no problem selling them.
These products are expensive but retailers have no problem selling them.
You can see that although they are very similar in meaning, although and but are with different parts of the sentence. As illustrated here, we use but to show an unexpected result or a result in contrast to our expectation, whereas although is used to show a situation that has a surprising result.
In our example here, these products are expensive is the situation while retailers have no problem selling them is the result.
Another difference that I can think of right now is that but is a conjunction found in a compound sentence while although is a conjunction of a complex sentence. That's why some people believe that we can start a sentence with although but not with but.
I like to give the following quotation to those who ask me whether But can begin the sentence:
Sentences beginning with "and", "but", or "or" are acceptable provided the practice is not overdone. Indeed, nothing that is overdone is ever acceptable, be it language or lamb chops.
"”Theodore M. Bernstein