The traditional convention is to conclude with 'Yours sincerely' if you began with the person's name ( e.g. Dear Mr Brown), and to conclude with 'Yours faithfully' if you began without using person's name ( e.g. 'Dear Sir' ). It is all to do with levels of formality.

They mean the same thing. They are both polite ways to end a correspondence of some sort and there are others as well. There is no one proper way.

thank you.it is very helpful

I suggest using "yours faithfully" for people with a higher authority than you, eg. if you're applying for a job. Just look up "faithful" and "sincere" and you'll get the idea.

If you know the name of the person you are writing to you end the letter Yours sincerely. If you don't know the person's name you end Yours faithfully. Both ways are formal and you wouldn't write either if you are corresponding with a friend.