When writing a business letter or marketing letter, a valediction used to say farewell.

"Yours sincerely" is typically employed 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 known by name (i.e. the recipient is addressed by a phrase such as "Dear Sir/Madam"). One way to remember this is the saying 'S and S never go together' (for Sir and Sincerely respectively). There is the option when the recipient not been previously met or spoken with, to use of the more formal 'Yours faithfully'.

"Yours truly" is used in professional correspondence, e.g. by solicitors and accountants, 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. It is also seen more regularly in American English.

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References

Wikipedia Article on Valedicitons

Marketing and introductory letter writing services page