If you mean the letter that you have to show the UK Border Agency to prove that you have accommodation, it isn't the same thing but you may need your bank statement as part of the same process. The letter from your landlord or wherever you are staying is to prove that you have a place to stay; your bank statement could be asked for, to prove that you have enough money for your course or other activity - ie, you won't be illlegally looking for work.
So you should have both forms of document handy before you start the application process, as well as your passport, acceptance letter from your college or similar, and any other ID.
You may actually not need to show all of this, especially if it's clear that you are enrolled on a course of study, but they do have the right to ask for it all so it's best to be prepared. You can find the answers to a lot of questions here, and if you contact the agency directly they should be able to help. Good luck with your application!