"Yours sincerely", "Yours faithfully" or “Yours truly”

"Yours sincerely", "Yours faithfully" or “Yours truly”

"Yours sincerely" is typically used in British English when the recipient is addressed by name and is known to the sender to some degree, whereas "Yours faithfully" is used when the recipient is not addressed by name, e.g., "Dear Sir/Madam". When the recipient's name is known, but not previously met or spoken with, some people prefer the use of the more distant “Yours faithfully”, at the risk of annoying the recipient.

In American English, "Sincerely yours" or "Sincerely" is commonly used in formal correspondence, and "Faithfully yours" is rare. "Best wishes" and "Best regards" are also common in formal situation. In contrast to British English, there is no special convention for combining these with any particular salutation.

“Yours truly” can carry either or both of these two connotations. "Yours truly" is also used in professional correspondence when writing to a client by his name, but signing the letter in the name of the firm where neither "Yours faithfully" or "Yours sincerely" would be appropriate.

Further readings:
http://en.wikipedia.org/wiki/Valediction