A help desk is a place where you can get information and assistance with technical problems you might have.  Many of them are broken off into two categories, the ticket portion and the tutorial portion.

In the ticket portion, you have a way for customers to contact a company or business, to answer a question or to get a response.  The tutorial portion often has FAQs (frequently asked questions) answered, and popular help articles that give information for topics that are most asked about.

As for the history of the modern help desk, here is a little more information about that from Wikipedia:

In the mid 1990s, Middleton at Robert Gordon University found through his research that many organizations had begun to recognize that the real value of their help desk(s) derives not solely from their reactive response to users’ issues but from the help desk’s unique position where it communicates daily with numerous customers or employees. This gives the help desk the ability to monitor the user environment for issues from technical problems to user preferences and satisfaction. Such information gathered at the help desk can be valuable for use in planning and preparation for other units in IT.

So as you can see, at it’s core – any help desk’s purpose is to help the customer get more information, by any means necessary.